MCQ test practice bank

Question 1

What is its normal habitat?

Answer 1

• the upper respiratory tract.

Haemophilus influenzae may typically be isolated from the upper respiratory tract. In cases of meningitis, it gains access to the meninges via the bloodstream – the hematogenous route.

Question 2

Immunization is useful for the prevention of:

Answer 2

• pneumococcal pneumonia in particularly susceptible patients.
• whooping cough.
• tuberculosis

The BCG vaccine protects against tuberculosis, and the DPT vaccine provides cover against the causative agents of diphtheria, whooping cough and tetanus. Haemophilus influenzae vaccines are available but do not provide protection against all the strains that can cause bronchopneumonia. Pneumococcal vaccines are now available.

Question 3

What properties make this bacterium especially likely to cause prosthetic valve endocarditis?

Answer 3

• It is a skin commensal;
• It produces extracellular slime.

The coagulase-negative staphylococci are common skin commensals. As such, they may cause infections when introduced accidentally into surgical wounds. They frequently cause infections when devices are implanted. This is because many strains produce large amounts of extracellular slime. This acts as a type of microbial glue, enabling the bacteria to stick to the implants in a biofilm. This makes antibiotic treatment very difficult as antibacterial drugs penetrate biofilms very poorly. Consequently, when implants are infected, in many cases treatment will only be effective once the affected device is removed or replaced. Although biofilms make antibiotic therapy difficult and although the susceptibility of coagulase-negative staphylococci to antibiotics may be difficult to predict, this is not relevant in their role in causing endocarditis.

Question 4

The following are examples of exogenous infection:

Answer 4

• cholera
• typhoid fever

Both typhoid fever and cholera are water-borne infections and so are exogenous. Dental caries results from the metabolism of sugars by the oral microbiota to produce acid that etches the enamel of teeth, producing carious lesions. ‘Honeymoon cystitis’ results from the introduction of Staphylococcus saprophyticus into the female urinary tract during sexual intercourse. Endocarditis is caused by many microorganisms, typically derived from the commensal microbiota.

Question 5

Legionnaire’s disease is diagnosed by:

Answer 5

• a rising antibody titer over 14-21 days.

The diagnosis of Legionnaire’s disease relies upon a demonstration of a rising antibody titer over a 14-21 day period. A seven-day interval between tests is too short to demonstrate a clear difference in antibody titers between the acute and the convalescent samples. Although the causative agent, Legionella pneumophila, can be grown in artificial culture, it is fastidious and can be difficult to isolate. Urine is an inappropriate sample for culture examination in this disease.

Question 6

The following pathogens have humans as their reservoir:

Answer 6

• Corynebacterium diphtheriae
• Salmonella Typhi
• Vibrio cholerae

Corynebacterium diphtheriae, Salmonella Typhi and Vibrio cholerae are all pathogens that have their reservoir in human populations. Salmonella Typhimurium is associated with animals and Clostridium tetani are found in soils (and the guts of many domesticated animals, but not humans).

Question 7

The following survive(s) inside human cells and typically cause(s) systemic infection involving multiple organs known as enteric fever:

Answer 7

• Salmonella Typhi.

Salmonella Typhi is an intracellular parasite that causes enteric fever, a generalized infection that affects multiple organ systems. Salmonella Typhimurium produces a similar illness when it infects mice (as the name indicates) but typically causes gastroenteritis in humans; it is only invasive in people of fragile health. Shigella dysenteriae causes dysentery, sometimes known as the “bloody flux”. Vibrio cholerae is the cause of cholera. This illness is characterized by the production of a toxin in the gut that reverses the sodium pump, resulting in a massive fluid loss. Enterobacter cloacae is a hospital pathogen that causes opportunistic infections that are frequently difficult to treat because of the antibiotic resistances that these bacteria may express.

Question 8

The following test(s) help in the diagnosis of Pneumocystis jiroveci pneumonia:

Answer 8

• bronchial lavage and microscopy.

Pneumocystis jiroveci pneumonia is one of the opportunistic infections that a transition from HIV infection to AIDS. The causative pathogen was once classified as a protist but is now recognized as a fungus. Bronchial lavage followed by microscopy is used to detect ‘cysts’. This may be helped by modifying the Ziehl-Neelsen or auramine-rhodamine staining protocols. It does not grow on conventional laboratory media.

Question 9

What selective medium could be used to isolate this bacterium?

Answer 9

• neomycin fresh blood agar;

Gangrenous lesions are typically polymicrobial. In such cases, it is necessary to apply selective pressure to isolate the primary pathogen. In gangrene, the pathogen is an obligate anaerobe. As such, it is intrinsically resistant to the aminoglycoside antibiotic neomycin. Most other bacteria in gangrenous lesions are susceptible to this antibiotic. The inclusion of neomycin in fresh blood agar thus makes a good selective medium for this pathogen. Use of fresh blood in the agar is useful for another reason. The primary pathogen is hemolytic and so its colonies may easily be seen by a zone of clearing around each one growing on a medium containing fresh blood.

Question 10

JC is 10 years old. He has just started boarding school and who rapidly developed pyrexia, headache, and convulsions. The onset of these symptoms was accompanied by the appearance of a petechial rash. A sample of his cerebrospinal fluid revealed 4.0 x 107 leukocytes per liter. The majority of these were polymorphs. No red cells were seen. The protein concentration was 2.3 g/l (normal range 0.14 to 0.45 g/l) and the sugar was 1.4 mmol/l. This compares with a blood sugar level of 4.6 mmol/l. A Gram stain revealed the presence of bacteria – intracellular Gram-negative diplococci within the cytoplasm of polymorphonuclear leukocytes (neutrophils). On incubating a chocolate plate inoculated with the CSF from this patient under 10% CO2, Isolate ‘E’ was made. Nothing grew when the CSF was cultured in air. The isolate is a Gram-negative diplococcus that was isolated under elevated CO2 is oxidase-positive.

Answer 10

• Neisseria meningitidis

Given the Gram-reaction, the requirement for an elevated level of carbon dioxide for artificial culture, the intracellular location and the fact that the patient is suffering a characteristic petechial rash, the identity of this pathogen is Neisseria meningitidis.

Question 11

How may this bacterium have gained access to the bloodstream?

Answer 11

• Through DB’s prostate gland;

A transurethral resection is a very common operation for aging men. It is used to relieve the symptoms of prostatic enlargement and some of the prostate gland tissue is removed, leaving an open wound. It is via this damaged tissue that the bacterium has gained entry to the patient’s bloodstream in this case. Although DB’s urinary tract was the focus of this operation, there is nothing in the information given to suggest that he has a problem with his kidneys. Likewise, there is no suggestion that the patient’s bowel integrity was disrupted during the operation.

Question 12

Given the information now available, the identity of this isolate is…

Answer 12

• a ‘viridans’ streptococcus

The fact that this bacterium is catalase-negative indicates that it is a streptococcus rather than a staphylococcus. The resistance to optochin and bile insolubility differentiates this “viridans” streptococcus from Streptococcus pneumoniae, another α-haemolytic streptococcus. Streptococcus pyogenes is β-haemolytic. During M’s evaluation, it was noted that her heart murmur, associated with a congenital valve defect, had changed. What is the explanation for this change? As the bacteria grow on damaged her heart valve, they stimulate the formation of “vegetations” by causing fibrin to be deposited. This causes further disruption of the blood flow through the valve, causing the murmur to change.

Question 13

A culture of a fecal sample growing aerobically on MacConkey agar

The culture illustrated was made by streaking a feces sample onto MacConkey agar and incubating aerobically overnight. Two colony types are apparent, a large, red colony and a small, red colony.

The Gram film prepared from the larger of the colonies isolated from the fecal sample

This bacterium is a Gram-negative bacillus.

Growth on MacConkey agar results in large red colonies, indicating that this bacterium is a lactose fermenter.

The identity of this bacterium is:

Answer 13

- Escherichia coli

Question 14

Tuberculosis:

Answer 14

• is a recognized cause of infertility in women.
• in children is often asymptomatic.
• in elderly patients is usually acquired from another individual with “open” tuberculosis.
• may be acquired by drinking unpasteurized milk.

Although this is very rare in countries with programmes for the screening and elimination of cattle infected by Mycobacterium bovis, tuberculosis may be a consequence of drinking unpasteurized milk. BCG vaccine is a live attenuated vaccine and is not heat killed. In a large study of 701 women with tuberculosis of the genital tract, infertility was the commonest presentation. Most infections in the elderly are reactivations of primary infection.

Question 15

Microscopical examination of an appropriate specimen (e.g. sputum, swab, secretions) may permit the diagnosis of:

Answer 15

• Vincent’s angina.
• pneumococcal lobar pneumonia.
• tuberculosis

The presence of encapsulated lanceolate Gram-positive diplococci in purulent sputum is diagnostic of pneumococcal pneumonia, and the presence of acid-alcohol fast bacilli in specimens stained by the Ziehl Neelsen method is indicative of tuberculosis. Care must be taken when examining urine because the saprophyte Mycobacterium smegmatis may be confused with the pathogenic Mycobacterium tuberculosis. A Gram film of a throat swab showing the presence of fusiform bacilli associated with spirochaetes is diagnostic of Vincent’s angina. The presence of “atypical” monocytes in peripheral blood films may be suggestive of glandular fever, but they may be seen in other conditions as well. It is not possible to distinguish Bordetella pertussis from members of the human normal microbiota upon microscopy.

Question 16

The following may be spread by infected blood products:

Answer 16

• hepatitis B virus
• hepatitis C virus
• human immunodeficiency virus
• Treponema pallidum

Hepatitis A virus spreads via the fecal-oral route. All of the other pathogens, including Treponema pallidum, the spirochaete that causes syphilis, may spread through infected blood or blood products.

Question 17

The following may be part of the commensal microbiota of the vagina of a post-menopausal woman:

Answer 17

• Enterococcus faecium
• Escherichia coli

The commensal microbiota of a post-menopausal vagina resembles the fecal microbiota. Staphylococcus saprophyticus is a skin commensal that causes “honeymoon cystitis”, an infection of the urinary tract rather than the genital tract. Lactobacilli dominate the commensal microbiota of the vagina in women of reproductive age.

Question 18

Bacteria responsible for community-acquired pneumonia include:

Answer 18

• Streptococcus pneumoniae
• Legionella pneumophila
• Mycoplasma pneumoniae

The commonest cause of community-acquired pneumonia is Streptococcus pneumoniae. Legionella pneumophila is an uncommon but important cause of his disease. Mycoplasma pneumoniae is another common cause of community-acquired pneumonia. Chlamydia trachomatis is a rare cause of hospital-acquired pneumonia in neonates. However, Chlamydia pneumoniae, a newly recognized organism, is increasingly recognized as a cause of community-acquired pneumonia. Pseudomonas aeruginosa lung infections are associated with ventilators used in hospitals to assist breathing.

Question 19

Sore throat with fever:

Answer 19

• when recurrent is an indication for tonsillectomy.

Half or more cases of sore throat with fever are due to viral infection, and not Streptococcus pyogenes. In many cases, treatment may be deferred until the results of tests for streptococcal infection are available. A rise in antibody titer may not occur for two weeks or more following infection. Streptococcal sore throat is classically associated with rheumatic fever. Acute glomerulonephritis is also associated with streptococcal impetigo.

Question 20

The following may be spread by the fecal-oral route:

Answer 20

• Shigella sonnei
• Vibrio cholerae

Vibrio cholerae and Shigella sonnei are spread via the fecal-oral route. Both rhinoviruses, which cause the common cold, and Neisseria meningitidis, the cause of meningitis, are spread through inhalation of infected droplets. As a venereal pathogen, Neisseria gonorrhoeae, the cause of gonorrhea, is spread during sexual intercourse.

Question 21

ED is eight months old. She had suffered badly from eczema since birth. Because of this, her parents were advised not to have her vaccinated with the Hib vaccine at two months. She became unwell, and her family doctor arranged to admit her to the hospital because she was suffering from pyrexia and she was restless and off her feeds. A lumbar puncture was performed and Isolate ‘F’ was obtained upon the culture of the CSF. This bacterium is a pleomorphic Gram-negative bacillus that grows in the air but that is highly fastidious, requiring the presence of both the “X” and “V” growth factors - haem and NAD, respectively.

Answer 21

• Listeria monocytogenes

The features of this bacterium, particularly its nutritional requirements, indicate that it is Haemophilus influenzae. Were it to be subjected to strain typing, it would almost certainly belong to the Pittman type B.

Its identity may be confirmed by its dependence upon X and V growth factors (haem and NAD respectively). Alternatively, satellitism of colonies around Staphylococcus aureus growing on fresh blood agar will demonstrate nutritional dependence.

Babies are routinely protected from this infection by application of the Hib vaccine. In this case, however, vaccination had been contra-indicated, with disastrous consequences.

Question 22

Recently, a molecular biological view of virulence and disease has arisen. Observations that link a gene with a virulence trait include:

Answer 22

• the gene in question must be translated.
• if disrupted, the gene encoding virulence converting the microbe from a virulent isolate to an avirulent form.
• introduction of the gene being tested into a strain of the same species that are not able to cause disease transforming the recipient strain to virulence.
• antibodies raised against the gene product being tested affording a degree of protection from illness.

To decide what genes code for factors that increase the ability of a microorganism to cause disease, the following should be demonstrated:

• the gene encoding the trait of interest should be present, transcribed and translated in a virulent strain;
• the gene encoding the trait of interest should NOT be present or should be silent in a strain that does not cause disease;
• disruption of the gene in a virulent strain should result in the formation of a strain that is incapable of causing disease;
• introduction of the gene into a strain that previously did not cause disease should transform the strain into one that does cause disease; N.B. some virulence traits may require the expression of more than one gene;
• the gene must be expressed during infection; antibodies raised against the gene product or the appropriate cell-mediated immunity should protect experimental subjects against disease.

Question 23

Concerning dental caries:

Answer 23

• this may result in osteomyelitis.
• this may result in a life-threatening condition.
• this may regress in the early stages of the disease.
• this may result in the formation of a brain abscess.

Although Streptococcus mutans plays a central role in the initiation of dental caries by providing an attachment for many other bacteria that contribute to cavity formation, dental caries is a complex polymicrobial process. In the very early stages, dental caries may regress but in most cases, the treatment is to “drill and fill”. Life-threatening conditions including brain abscesses and endocarditis may result from dental caries. Untreated, another complication of dental caries may be osteomyelitis in the adjacent bones.

Question 24

Compared with people in the wider community, people who are hospitalized are more likely to have urinary tract infections caused by:

Answer 24

• Proteus mirabilis.
• Staphylococcus aureus.
• Candida albicans.

Causes of urinary tract infections in hospital in-patients:

• Escherichia coli : 40%
• coagulase-negative staphylococci: 3%
• ‘other’ Gram-negative bacteria: 25%
• ‘other’ Gram-positive bacteria: 16%
• Candida albicans : 5%
• Proteus mirabilis : 11%

Causes of urinary tract infections in the wider community:

• Escherichia coli : 80%
• coagulase-negative staphylococci: 7%
• ‘other’ Gram-negative bacteria: 4%
• ‘other’ Gram-positive bacteria: 3%
• Proteus mirabilis : 6%

Thus, Candida albicans, Staphylococcus aureus, and Proteus mirabilis are over-represented as causes of urinary tract infections in hospital in-patients compared with people in the wider community.

Question 25

Chlamydia:

Answer 25

- infections respond to tetracycline. - cells contain DNA and RNA. - infections are increasingly being diagnosed. Chlamydial lung infections are characteristically bilateral. Although chlamydia is obligate intracellular parasites, they are bacteria, and they contain both DNA and RNA, unlike viruses which have only one type of nucleic acid. They lack cell walls, and so chlamydial infections cannot be treated with β-lactam antibiotics. However, they do respond to tetracycline therapy. Chlamydia is increasingly being recognized as human pathogens.

Question 26

**The gram film prepared from the smaller red colony isolated from the fecal sample** It is a Gram-positive coccus that tends to form chains. Subsequent testing shows that this bacterium is not capable of releasing oxygen bubbles from a dilute solution of hydrogen peroxide, indicating that it is **catalase negative**. This isolate grows on MacConkey agar producing acid from the fermentation of lactose. It is thus considered to be a lactose fermenter. When this bacterium is plated onto fresh blood agar it does not alter the appearance of the blood cells. The identity of this bacterium is:

Answer 26

- Enterococcus faecalis The stomach is considered to be free of commensal microbiota. This is because the extreme acid pH at this site is inhibitory. The small intestine is increasingly colonized as it reaches the large intestine. The concentration of bile at the proximal end is inhibitory to many microbes. The fecal microbiota has been estimated to have a metabolic capacity equal to that of the liver. Obligate anaerobic bacteria, such as those of the genus Bacteroides, outnumber the facultatively anaerobic coliforms, such as Escherichia coli (the commonest bacterium to grow aerobically from feces) by at least 100:1. Diet may have a profound effect on the composition of the gut microbiota. Bottle-fed babies have a fecal microbiota that is entirely different from that of breastfed babies. The former resembles the adult fecal microbiota: the latter is predominantly bifidobacteria. The microbiota of bottle-fed babies breaks own proteins efficiently; the microbiota of breast-fed babies breaks down sugars. The smell and appearance of the feces reflect these differences!

Question 27

Concerning sex bias in infections:

Answer 27

- men are more likely to suffer from leptospirosis than are women. - boys are more likely to suffer from streptococcal skin infections than are girls. Given the relative anatomies of the bowel and urinary tracts, it is women who are more likely to suffer from cystitis than are men but because infant boys are more likely to suffer from anatomical abnormalities that cause partial obstruction of the urinary tract, they are more likely to suffer from bladder infections than are infant girls. Boys are more likely to suffer skin abrasions leading to streptococcal skin infections than are girls. Occupational exposure increases the risk of leptospirosis in men compared with women; a disease associated with contaminated rat urine. There is no sex bias seen in meningococcal meningitis.

Question 28

What feature of the bacterium is responsible for the clinical condition?

Answer 28

- Endotoxin encoded by the bacterial chromosome. DB is suffering from endotoxic shock. Its symptoms include rigors, rapid heartbeat, and breathing and low blood pressure. Although the genes that code for exotoxins may be located on the bacterial chromosome or on a variety of plasmids (and even as part of the genome of a bacteriophage), the genes responsible for the expression of endotoxin in Gram-negative bacteria are always located on the chromosome. This is because endotoxin is an integral part of the Gram-negative outer membrane.

Question 29

Before therapy for syphilis, a patient tested positive in both the RPR and TPPA tests. Assuming that therapy was successful, what pattern of test results would you predict after treatment?

Answer 29

- the RPR test will become negative but the TPPA test will remain positive. The RPR test is a "non-specific" test. It relies upon the flocculation of cardiolipin antigen with antibodies in a positive serum. The test is enhanced by the addition of carbon particles, which aid the visualization of the flocculation. Although it is a non-specific test, it is useful in the following treatment, since the antibody titre declines on successful therapy. Thus, a patient who has been treated successfully will have a negative RPR test. In the TPPA test, specific treponemal antigens are attached to gelatin beads. Unlike the RPR test, it is a specific test for treponemal infection. The treated beads are agglutinated by antibodies in a positive serum. The use of control beads in each test helps to identify false positive reactions. As a specific test of treponemal infection, once a patient has a positive TPPA test, this will remain positive for life.

Question 30

Mycobacterium avium-intracellulare:

Answer 30

- is a common opportunistic bacterial pathogen in patients with AIDS. - causes swollen lymph nodes in the neck in children. Members of the Mycobacterium tuberculosis complex include Mycobacterium tuberculosis, Mycobacterium bovis, Mycobacterium africanum and Mycobacterium microti, but not Mycobacterium avium intracellulare. It is the commonest cause of cervical lymphadenopathy in children in western countries and causes disseminated infections only in severely immunocompromised patients. It is a common opportunistic bacterial pathogen in patients suffering from AIDS. It is characterized by the production of non-pigmented, slow-growing colonies.

Question 31

**The gram film prepared from the greenish colony isolated from the throat swab** It is a Gram-positive coccus that tends to form chains. Subsequent testing shows that this bacterium is not capable of releasing oxygen bubbles from a dilute solution of hydrogen peroxide, indicating that it is catalase negative. This isolate causes a change in the appearance of the blood surrounding the colony. It causes partial hemolysis, referred to as a-hemolysis, giving rise to a green halo around its colonies. The identity of this bacterium is:

Answer 31

- a "viridans" streptococcus. The oral cavity and throat have a rich and varied microbiota. There are marked differences between the different surfaces of the oral cavity. The bacteria that attach to the hard enamel of the teeth are different in type from those that colonize the soft tissues that are in intimate anatomical contact with the teeth. There are many anaerobes that live in this area, and protists can also frequently be found. Both Gram-positive and Gram-negative bacteria are commonly seen in this area. The predominant bacteria are alpha-hemolytic streptococci. Because they are surrounded by a greenish halo when grown on blood agar they are sometimes known as 'viridans streptococci'. Some people carry pathogenic bacteria as a part of the commensal microbiota of the throat. These include Streptococcus pneumoniae, Streptococcus pyogenes, and Neisseria meningitidis.

Question 32

RS is a 46-year-old man with a history of chronic alcoholism. He was admitted to the hospital and gave a history of headache and vomiting. He was pyrexial, photophobic and had neck stiffness. Isolate 'G' was isolated from his CSF. It is a Gram-positive coccus that cannot release oxygen from hydrogen peroxide. It is also susceptible to optochin. Its colonies are surrounded by a greenish halo when growing on fresh blood agar. What alterations have been made to the growth medium?

Answer 32

- α-haemolysis

Question 33

Is this different from the bacteria that constitute the normal microbiota from sites within the body?

Answer 33

- Yes The skin is often dry compared with other body surfaces and has a very high salt content. This makes it an osmotically hostile environment. As such, it is not suitable for organisms that are osmotically fragile. In consequence, most of the bacteria found in the skin microbiota are Gram-positive, with robust cell envelopes. Elsewhere on the body, osmotic stress is not a problem and Gram-negative bacteria are just as likely to be found as are Gram-positive bacteria, although other factors may influence the relative distribution of different bacterial types.

Question 34

Towards the end of her three-week stay, a 17-year-old student who visited England as part of a school exchange visit noticed that she was suffering from a vaginal discharge. On the following day, her left knee was swollen and red. It was very painful and was hot to touch. Aspiration of the synovial fluid yielded a specimen of frank pus. Microscopic examination showed the presence of Gram-negative intracellular diplococci within polymorphonuclear leukocytes. At a later consultation, she said that shortly after arrival in England, she had had intercourse and that for a number of days before her joint problems she had noticed a purulent vaginal discharge. The culture of the knee aspirate is likely to yield:

Answer 34

- Neisseria gonorrhoeae. Although Chlamydia trachomatis, Neisseria gonorrhoeae, and Ureaplasma urealyticum have all been associated with arthritis, the observation of Gram-negative diplococci in the knee aspirate indicates that this case is caused by Neisseria gonorrhoeae.

Question 35

In non-specific urethritis:

Answer 35

- the most common causative bacterium is an obligate intracellular parasite. - the condition may be associated with reactive arthritis. - the most common causative bacterium may cause co-infection with Neisseria gonorrhoeae - the condition may be associated with an eye infection. The most common cause of non-specific urethritis, also known as non-gonococcal urethritis is Chlamydia trachomatis. Despite the alternative name for the infection caused by this bacterium, it commonly causes co-infection with Neisseria gonorrhoeae. Chlamydia trachomatis is an obligate intracellular parasite that cannot be grown on conventional laboratory media. It grows in McCoy cells and may be detected by DNA amplification. A common complication of non-specific urethritis is an eye infection, caused by autoinoculation with Chlamydia trachomatis. Much more rarely, a chlamydial infection may lead to Reiter's syndrome, reactive arthritis associated with people with the HLA-B27 tissue type.

Question 36

The following may be found as part of the commensal microbiota of the colon when cultivated in the air:

Answer 36

- Enterococcus faecium - Escherichia coli Corynebacteria and Staphylococcus saprohyticus are not found as commensals in the colon. Bacteroides spp are obligate anaerobes, and cannot be grown in air. Enterococcus faecium and Escherichia coli grow aerobically. The species names of both of these bacteria reflect their natural habitat: the gut. Remember, however, these species will be outnumbered by anaerobic bacteria by at least 100:1.

Question 37

What is the identity of the causative agent?

Answer 37

- Clostridium perfringens All of these pathogens can cause skin infections, although the obligately aerobic Pseudomonas aeruginosa requires the skin to be damaged, for example by a burn. Bacillus anthracis causes anthrax, the lesions of which are shiny and black. In this case, the patient is suffering from gas gangrene. This is caused by Clostridium perfringens . The Nagler test is used to identify this bacterium. An egg-yolk plate is divided into two halves. Over one half is spread a specific antitoxin. The culture to be tested, together with the positive control, if available, is streaked across the plate, going from the untreated area of the plate to the area that is covered with antitoxin. The culture is then incubated anaerobically. Following incubation, lecithinase activity, caused by the action of the Clostridium perfringens a -toxin is seen as a precipitate in the medium on the side of the plate that did not receive the antitoxin, but not on the side that was treated.

Question 38

Acute exacerbation of chronic bronchitis is associated with the presence of the following bacteria in the lower respiratory tract:

Answer 38

- Haemophilus influenzae - Streptococcus pneumoniae Haemophilus influenzae and Streptococcus pneumoniae are associated with acute exacerbations of bronchitis. Streptococcus pyogenes causes severe throat infections. Bordetella pertussis is the causative agent of whooping cough, and Staphylococcus aureusmay cause pneumonia.

Question 39

The following may cause illness by intoxication:

Answer 39

- Staphylococcus aureus - Bacillus cereus - Aspergillus flavus - Clostridium botulinum - Claviceps purpurea The fungi Aspergillus flavus and Claviceps purpurea produce aflatoxins. Aflatoxin, produced by Aspergillus flavus is a potent carcinogen; ergotamine, the product of Claviceps purpurea releases LSD when metabolized and is a potent hallucinogen. Staphylococcus aureus, Bacillus cereus, and Clostridium botulinum all cause food poisonings. In the case of Staphylococcus aureus and Bacillus cereus, these are relatively trivial illnesses involving profuse vomiting for a limited period. Clostridium botulinum causes flaccid paralysis that carries a high risk of mortality.

Question 40

The following is NOT one of Koch's postulates:

Answer 40

- the pathogen causes antibodies to be raised in infected individuals. Although these days one would expect antibodies to be raised against the causative organism in natural cases and in individuals infected experimentally, this requirement is not one of the postulates as stated by Robert Koch.

Question 41

Koch's postulates are:

Answer 41

- the microbe must be recovered from an experimentally infected host. - the disease must be reproduced when a pure culture is introduced into a non-diseased, susceptible host. - the microbe is present in every case of the disease. - the microbe must be isolated from the diseased host and grown in pure culture. It is not one of Koch's postulates that a disease cannot be caused by more than one pathogen. There are examples when this does not occur e.g. pneumonia. The combination of the four postulates implies that a specific microbe causes a specific disease.

Question 42

Clinical manifestations of osteomyelitis include:

Answer 42

- warmth around the affected area. - redness around the affected area. - systemic upset - fever, chills, night sweats. - pain and swelling over the affected area. In osteomyelitis, patients demonstrate reduced mobility of the affected area, rather than hypermobility.

Question 43

The following bacteria may be isolated as part of the commensal microbiota of the oral cavity:

Answer 43

- Neisseria spp. - Streptococcus mutans - Streptococcus salivarius​ - Moraxella spp.

Question 44

Concerning the laboratory diagnosis of syphilis:

Answer 44

- once positive, the FTA(abs) test remains so for life. - once positive, the TPPA test remains so for life. - screening of donated blood is routine in the United Kingdom to prevent the horizontal spread of the disease. Treponema pallidum will not grow on conventional laboratory media. While animals may be used for the propagation of Treponema pallidum in the laboratory, the usual model is the eyes or testes of rabbits; the nine-banded armadillo is used for the propagation of Mycobacterium leprae. Screening of blood for syphilis is used for all donations in the United Kingdom . The diagnosis of syphilis typically relies on serological testing. The tests may be specific for treponemal antigens or non-specific, seeking antibodies to antigens other than those expressed by Treponema pallidum. Once positive, the specific antigen tests such as FTA(abs), the fluorescent treponemal antibody absorption test, and the TPPA - Treponema pallidum particle agglutination test - remain positive for life. Non-specific tests such as the rapid plasma reagin (RPR) test become negative following successful treatment of the infection. Such non-specific tests are useful in the monitoring of therapy.

Question 45

Osteomyelitis may be caused by:

Answer 45

- Staphylococcus aureus - Streptococcus pyogenes - Klebsiella pneumoniae - Mycobacterium tuberculosis All of the pathogens listed may cause osteomyelitis apart from Bordetella pertussis, the cause of whooping cough.

Question 46

The gut microbiota provides protection against infection caused by:

Answer 46

- Clostridium difficile Clostridium difficile causes antibiotic-associated diarrhea. Following administration of an antibiotic, particularly one that is excreted via the gall bladder, the commensal microbiota of the gut becomes disturbed. This provides toxigenic strains of Clostridium difficile with the opportunity to flourish and cause disease. This may range from relatively mild diarrhea to the potentially fatal pseudomembranous colitis and rupture of the colon. This illustrates the role of the commensal microbiota in preventing disease. Clostridium acetobutylicum is used in the production of acetone from glucose. It was Chaim Weizmann's work with this bacterium that led to the establishment after the Second World War of Israel as an independent state. Clostridium botulinum is the cause of a flaccid and often fatal flaccid paralysis known as botulism. Clostridium perfringens may be isolated as a gut commensal and Clostridium tetani, a soil bacterium, causes tetanus, a disease in which muscles go into spasm.

Question 47

Legionnaire's disease:

Answer 47

- often presents as atypical pneumonia characterized by high fever and confusion. - is named after a convention of First World War veterans. - causes deaths in the UK. Legionnaire's is so named after the 58th annual convocation of the First World War veterans, held in the Hotel Bellevue, Stratford in Philadelphia in 1976. Over 200 delegates acquired severe pneumonia and 30 died. The causative agent is a Gram-negative cocco-bacillus, but the treatment of choice is erythromycin, an agent that is typically only active against Gram-positive bacteria. It is a fastidious bacterium, and will only grow on media rich in iron and cysteine, and containing activated charcoal, used to adsorb its toxic metabolic products. The disease is derived from inhalation of infected water droplets and is typically associated with dirty shower heads or Jacuzzis, not conventional bath taps. This is because taps do not produce air-borne water droplets of a size that will facilitate infection of the alveoli.

Question 48

Pneumonia caused by Streptococcus pneumoniae:

Answer 48

- is a complication of splenectomy. - is usually caused by bacteria that can be isolated from the respiratory tract of healthy adults and children. - is defined as a bacterial infection of the lung parenchyma. Definitive diagnosis of streptococcal pneumonia relies upon the detection of the bacterium in blood cultures. In the United Kingdom, the vast majority of strains remain sensitive to penicillin.

Question 49

In acute uncomplicated urinary tract infection:

Answer 49

- numerous white blood cells are present in the urine. - E. coli is the most frequently encountered pathogen. - the bacterial count is usually \>105 organisms/ ml of urine. Surveys have shown that overall about 80% of UTIs are caused by E. coli and infections are seen more frequently in women. The criterion of \>105 organisms/ml is a reliable measure of significant bacteruria which with significant pyuria (\>10 WBC/cu.mm) invariably indicate an infection. However, counts of 104/ml and even 103/ml may be significant if pyuria and symptoms are present.

Question 50

The following may cause a flaccid paralysis:

Answer 50

- Clostridium botulinum Clostridium botulinum causes flaccid paralysis that carries a high risk of mortality. Staphylococcus aureus, Bacillus cereus, and Clostridium botulinum all cause food poisonings. In the case of Staphylococcus aureus and Bacillus cereus, these are relatively trivial illnesses involving profuse vomiting for a limited period. The fungi Aspergillus flavus and Claviceps purpurea produce aflatoxins. Aflatoxin, produced by Aspergillus flavus is a potent carcinogen; ergotamine, the product of Claviceps purpurea releases LSD when metabolized and is a potent hallucinogen.

Question 51

**Gram** **film prepared from the only colony isolated from a vaginal swab taken from a woman of reproductive age** It is a Gram-positive bacillus. It typically stains weakly and unevenly. Subsequent testing shows that this bacterium is **not motile** and is also not capable of releasing oxygen bubbles from a dilute solution of hydrogen peroxide, indicating that it is **catalase negative**. The identity of this bacterium is:

Answer 51

- a lactobacillus (Döderline bacillus)

Question 52

The following microbes do not fit Koch's postulate requiring an artificial culture of the pathogen:

Answer 52

- Mycobacterium leprae - Treponema pallidum Treponema pallidum and Mycobacterium leprae cannot be cultured on laboratory media, and thus cannot fulfill the requirement for artificial culture in Koch's postulates.

Question 53

The following may be spread by the respiratory route:

Answer 53

- rhinoviruses - Neisseria meningitidis Both rhinoviruses, which cause the common cold, and Neisseria meningitidis, the cause of meningitis, are spread through inhalation of infected droplets. Vibrio cholerae and Shigella sonnei are spread via the fecal-oral route. As a venereal pathogen, Neisseria gonorrhoeae, the cause of gonorrhoea, is spread during sexual intercourse.

Question 54

The following bacteria can cause serious respiratory disease in the community:

Answer 54

- Staphylococcus aureus.

Question 55

In acute osteomyelitis, holes are drilled into the bone:

Answer 55

- to provide a specimen for the laboratory. - to relieve pressure in the bone. - to debride the affected bone. - to reduce pain for the patient. Drilling holes in the bone will release the pressure caused by the accumulation of pus in the bone. This will allow the affected bone to be cleaned (debrided) and will provide easy access to a sample of pus for pathological examination. Most importantly for the patient, however, releasing the pressure will give instantaneous pain relief.

Question 56

Life-threatening infections caused by commensal bacteria include:

Answer 56

- endocarditis - gas gangrene - septicaemia - peritonitis Endocarditis may be caused either by oral bacteria settling on damaged heart valves or from skin bacteria introduced into the circulation during operations to replace damaged heart valves. Injecting drug users may suffer a particularly serious form of endocarditis caused by Staphylococcus aureus. Gas gangrene is caused by Clostridium perfringens, a gut commensal. Peritonitis is a condition in which the gut ruptures, permitting access of the gut commensals to the peritoneal cavity. Meningitis caused by commensal bacteria may occur when, for example, plastic 'shunts' are placed in the brain to relieve the symptoms of hydrocephalus. In such cases, the likely causative bacterium will be a coagulase-negative staphylococcus.

Question 57

Mycobacterium tuberculosis:

Answer 57

- when cultivated in vitro has complex growth requirements. Mycobacteria are non-sporing rods that are classified with the Gram-positive bacteria on the basis of their cell wall architecture. However, the hydrophobic cell wall of Mycobacterium spp (up to 60% of the dry weight is lipid) precludes staining by Gram's method. Other staining procedures e.g. Ziehl-Neelsen are therefore used. Mycobacterium tuberculosis is an obligate aerobe - even a small reduction in oxygen tension will hamper the growth of the bacterium. It is resistant to drying (it may survive in dried sputum for up to six months). Even under optimal conditions, Mycobacterium tuberculosis divides only every 18-24 hours. Thus it can take from six to eight weeks for a laboratory culture to grow. More rapid, semi-automated methods have reduced this time and PCR is often used to provide a rapid provisional diagnosis.

Question 58

**Culture, growing on fresh blood agar, of a high vaginal swab from a post-menopausal** The culture illustrated was made by streaking a vaginal swab from a post-menopausal lady onto fresh blood agar and incubating aerobically overnight. Two colony types are apparent, a large, buff colony and a small, buff colony. **Gram** **film prepared from a vaginal swab from a post-menopausal lady** In the Gram film, two different types of cell can be seen; one is a Gram-negative bacillus and the other is a Gram-positive coccus. The Gram-positive cocci form short chains. The Gram-negative bacilli show no particular arrangement. Gram film prepared from the larger of the colonies isolated from the post-menopausal vaginal swab This bacterium is a Gram-negative bacillus. Growth on MacConkey agar results in large red colonies, indicating that this bacterium is a l**actose fermenter**. The identity of this bacterium is:

Answer 58

- Escherichia coli

Question 59

In the acute sore throat:

Answer 59

- the disease is more common in the winter than summer. - the disease is very rarely acquired from contaminated food. Most sore throats have a viral etiology, but these cannot always be distinguished clinically from streptococcal sore throats. Sore throats tend to be more common in winter than in summer. Food is, admittedly very rarely, a vector for throat infections.

Question 60

Disease due to Mycoplasma pneumoniae:

Answer 60

- is treated with tetracycline or a macrolide antibiotic. - occurs in epidemics. Mycoplasma pneumoniae infections show a periodicity of about four years. Because these bacteria lack cell walls, they cannot be treated with β-lactam antibiotics such as the penicillins or cephalosporins. Instead, they succumb to treatment with tetracyclines or macrolides. These latter antibiotics inhibit bacterial protein synthesis.

Question 61

The Nagler reaction:

Answer 61

- is carried out on egg yolk agar. - is used to detect lecithinase production. - is used to identify Clostridium perfringens. The α toxin of Clostridium perfringens is a lecithinase, causing a white precipitate in egg-yolk medium, a rich source of lecithin. In the Nagler reaction, its activity is inhibited specifically by the presence on half the plate of anti-toxin.

Question 62

The commensal microbiota within the oral cavity is influenced to a major extent by:

Answer 62

- eruption of the first dentition. - tetracycline - amoxicillin The use of antibiotics, particularly those with a broad spectrum of activity, have a profound effect on the human commensal microbiota. Microbes have evolved to adhere to particular types of surface, and so when teeth first appear, this presents a new type of surface to which microbes become attached. This, in turn, leads to the mouth becoming colonized with a new array of microbes. It is the enamel on teeth that is important for this adhesion, and not whether the tooth is a first set or second set. False teeth provide yet another surface, providing for a different array of colonizers. Diet has little effect on the resident microbiota of the mouth, but it may influence the transient microbiota.

Question 63

Endocarditis following a dental extraction MT has a congenital heart valve defect and has recently undergone a dental extraction. Since this procedure, she has complained of excessive tiredness. Examination revealed low-grade pyrexia, changed heart murmur, anemia, and microscopic haematuria. 'Splinter hemorrhages' were apparent under the fingernails. M had not received any antibiotics recently. Three sets of blood cultures were taken over a 24-hour period. Two of the three sets yielded Isolate 'D' on subculture. It is a Gram-positive coccus that fails to release oxygen from hydrogen peroxide. Its small colonies are also surrounded by a greenish halo when growing on fresh blood agar. The change in color of the blood in the agar indicates that the bacterium is:

Answer 63

- α-haemolytic This bacterium happens to be catalase-negative but this does not have any influence on the appearance of its colonies on fresh blood agar. The greenish halo that surrounds each colony of this bacterium is indicative of **partial hemolysis** – also known as α-hemolysis. The green appearance around colonies is even more apparent when this bacterium is grown on heated blood agar but, in this case, the effect cannot be due to hemolysis, since heating the blood in HBA destroys the red blood cells that are the targets of hemolysis. The green color is thought to be the result of the partial breakdown of haem to give biliverdin, a green pigment. Complete hemolysis, in which bacterial colonies are surrounded by a zone of complete clearing, is referred to as β-hemolysis. To tell the difference between α- and β-hemolysis, just remember that "**beta is complete**".

Question 64

**Gram** **film prepared from the large flat colony isolated from the skin swab** It is a Gram-positive bacillus. It forms groups that are said to resemble 'Chinese letters'. Following liquid culture, these bacilli may be seen lined up like fence-posts. This arrangement is said to resemble a palisade. Subsequent testing shows that this bacterium is capable of releasing oxygen bubbles from a dilute solution of hydrogen peroxide, indicating that it is **catalase positive**. The identity of this bacterium is:

Answer 64

- Corynebacterium sp. The skin is a relatively dry environment. It also has an acid pH. It is thus inhospitable for many microbes. Most of the microbiota of the skin are Gram-positive bacteria such as the coagulase-negative staphylococci and corynebacteria. They can withstand drying better than other microbes. There are several species of coagulase-negative staphylococci and they are not often differentiated to species level. This bacterium is thus most accurately described as a coagulase-negative staphylococcus. Given the origin of the culture in this case, however, if it were to be identified to species level it would most likely be Staphylococcus epidermidis. Anaerobic bacteria can live within the pores of the skin. Propionibacterium acnes is an example. It metabolizes sebum, releasing free fatty acids. These enhance the antimicrobial environment provided by the skin. The eye has a specialized type of skin. It is kept clean by the washing action and inhibitory factors found in tears. In consequence, few microbes are found on the eye. Some people carry coryneform bacteria on their conjunctivae.

Question 65

Is this bacterium likely to be soluble in bile?

Answer 65

- yes

Question 66

The following may cause vivid hallucinations:

Answer 66

- Claviceps purpurea Claviceps purpurea produces the mycotoxin ergotamine that releases LSD when metabolized. This is a potent hallucinogen. Aflatoxin, produced by Aspergillus flavus, is a potent carcinogen. Staphylococcus aureus, Bacillus cereus, and Clostridium botulinum all cause food poisonings. In the case of Staphylococcus aureus and Bacillus cereus, these are relatively trivial illnesses involving profuse vomiting for a limited period. Clostridium botulinum causes flaccid paralysis that carries a high risk of mortality.

Question 67

Endotoxin:

Answer 67

- cause thrombosis. - causes fever. - causes vascular collapse. - equates to the lipid A component of lipopolysaccharide. Endotoxin consists of the lipid A component lipopolysaccharide from the outer cell membrane of Gram-negative pathogens. They are especially dangerous molecules to the host, causing a wide range of damage including vascular collapse, fever, hypoglycemia, thrombosis, iron depletion, reduced blood pressure.

Question 68

The Lübeck disaster in 1926, which resulted in the deaths of 76 children, was associated with the administration of a virulent strain of:

Answer 68

- Mycobacterium tuberculosis The Lübeck disaster was caused when a virulent strain of Mycobacterium tuberculosis rather than the vaccine strain was administered to 249 babies, resulting in the deaths of 76 children. Hepatitis B virus contaminated a vaccine used to protect American service personnel from yellow fever in 1942. While there are vaccines that afford some protection against leprosy, they are not accepted universally. There is, as yet, no vaccine against Plasmodium vivax.

Question 69

The following may be spread venereally:

Answer 69

- Neisseria gonorrhoeae

Question 70

In addition to being catalase-negative and α-hemolytic, this Gram-positive coccus is also resistant to the antibiotic optochin. Is it soluble in a solution of bile salts?

Answer 70

- No Being resistant to optochin, the isolate will also be insoluble in a solution of bile salts.

Question 71

The following is/are examples of exogenous infections:

Answer 71

- typhoid - tetanus - non-specific urethritis. Non-specific urethritis is caused by several pathogens, all of which are sexually transmissible. Salmonella Typhi is spread by the fecal-oral route and is thus an example of a pathogen that causes an exogenous infection. Tetanus is also an exogenous infection, caused when Clostridium tetani contaminate a wound, particularly if there is a compromised oxygen supply. Cystitis is most commonly caused by contamination of the urinary tract with bacteria from the bowel and hence represents an endogenous infection, as do most cases of endocarditis.

Question 72

**Culture** **of a throat swab growing on fresh blood agar** The culture illustrated was made by streaking a throat swab onto fresh blood agar and incubating aerobically overnight. Two colony types are apparent, a largish, buff-colored colony and a very small colony that has caused the surrounding medium to turn green. **Gram** **film prepared from the throat swab** In the Gram film, two different types of cell can be seen; both are round. One is Gram-positive, the other Gram-negative. Thus, this film contains both Gram-positive and Gram-negative cocci. The Gram-positive cocci are found in chains; the Gram-negative cocci are generally arranged in pairs and are said to be '**diplococci**'. G**ram** **film prepared from the largish, buff colony isolated from the throat swab** This bacterium is a Gram-negative coccus that is generally arranged in pairs and is said to be ' **diplococci**'. Subsequent testing shows that this bacterium is capable of turning a 1% solution of tetra methyl-para-phenylene diamine dihydrochloride, TMPPD, deep purple within five seconds of exposure to the reagent. It is thus considered to be **oxidase positive**. The identity of this bacterium is:

Answer 72

- a member of the family Neisseriaceae

Question 73

Sickle cell disease confers resistance to:

Answer 73

- malaria Sickle cell disease results from a single amino acid change in the β chain of hemoglobin. Consequently, hemoglobin precipitates and deforms erythrocytes, which are the site of infection of parasites of the genus Plasmodium. These cause malaria and cannot survive in sickle cells. Sickle cell disease is found in people who live in parts of the world where malaria is common.

Question 74

"Rice-water" stools are a characteristic of infection with:

Answer 74

- Vibrio cholerae "Rice-water" stools are characteristic of cholera, caused by Vibrio cholerae. "Chinese restaurant syndrome" is an intoxication caused by Bacillus cereus. E. coli O157 causes hemolytic uremic syndrome. Enteric fever is caused by Salmonella Typhi. Another name for dysentery, caused by Shigella dysenteriae is the "bloody flux".

Question 75

The "bloody flux" is associated with:

Answer 75

- Shigella dysenteriae Another name for dysentery, caused by Shigella dysenteriae is the "bloody flux". "Chinese restaurant syndrome" is an intoxication caused by Bacillus cereus. E. coli O157 causes hemolytic uremic syndrome. Enteric fever is caused by Salmonella Typhi. "Rice-water" stools are characteristic of cholera, caused by Vibrio cholerae.

Question 76

The following are examples of endogenous infection:

Answer 76

- "honeymoon cystitis" - dental caries - endocarditis Dental caries results from the metabolism of sugars by the oral microbiota to produce acid that etches the enamel of teeth, producing carious lesions. 'Honeymoon cystitis' results from the introduction of Staphylococcus saprophyticus into the female urinary tract during sexual intercourse. Endocarditis is caused by many microorganisms, typically derived from the commensal microbiota. Both typhoid fever and cholera are water-borne infections and so are exogenous.

Question 77

What incubation conditions would you use to culture this bacterium?

Answer 77

- in anaerobic culture. The causative pathogen, in this case, is an obligate anaerobe. Oxygen must be excluded from any artificial culture if it is to be successful.

Question 78

What selective medium could be used to isolate this (illness by intoxication) bacterium?

Answer 78

- neomycin fresh blood agar. Gangrenous lesions are typically polymicrobial. In such cases, it is necessary to apply selective pressure to isolate the primary pathogen. In gangrene, the pathogen is an obligate anaerobe. As such, it is intrinsically resistant to the aminoglycoside antibiotic neomycin. Most other bacteria in gangrenous lesions are susceptible to this antibiotic. The inclusion of neomycin in fresh blood agar thus makes a good selective medium for this pathogen. Use of fresh blood in the agar is useful for another reason. The primary pathogen is hemolytic and so its colonies may easily be seen by a zone of clearing around each one growing on a medium containing fresh blood.

Question 79

**Gram** **film prepared from the flat, dry-looking colony isolated from the nasal swab.** It is a Gram-positive bacillus. It forms groups that are said to resemble 'Chinese letters'. Following liquid culture, these bacilli may be seen lined up like fence-posts. This arrangement is said to resemble a palisade. Subsequent testing shows that this bacterium is capable of releasing oxygen bubbles from a dilute solution of hydrogen peroxide, indicating that it is **catalase positive**. The identity of this bacterium is:

Answer 79

- Corynebacterium sp. Many of the microbes that we encounter enter through the nose. Indeed, the nose has elaborate anatomy and nasal hairs, evolved to prevent potentially dangerous microbes from entering our lungs. The nasal microbiota resembles that of the skin in many respects. However, the air that we exhale is moist, and this makes the nose a more hospitable environment for Staphylococcus aureus, a bacterium that can cause serious infections as well as being a commensal.

Question 80

Haemolytic uraemic syndrome is caused by:

Answer 80

- E. coli O157. E. coli O157 causes hemolytic-uremic syndrome. "Chinese restaurant syndrome" is an intoxication caused by Bacillus cereus. Enteric fever is caused by Salmonella Typhi. Another name for dysentery, caused by Shigella dysenteriae is the "bloody flux". "Rice-water" stools are characteristic of cholera, caused by Vibrio cholerae.

Question 81

What incubation conditions would you use to culture this (Clostridium perfringens) bacterium?

Answer 81

- in anaerobic culture; The causative pathogen, in this case, is an obligate anaerobe. Oxygen must be excluded from any artificial culture if it is to be successful.

Question 82

"Atypical" mycobacteria:

Answer 82

- may produce pigment when grown in the absence of light. - can be stained by the Ziehl-Neelsen method. - can be found in domestic water supplies. - are often resistant to antituberculous drugs. Atypical mycobacteria can cause infections in previously healthy individuals e.g. "fish tank granuloma" caused by Mycobacterium marinum in tropical fish fanciers. Management of atypical mycobacterial infections may require the use of "second" or "third line" anti-tuberculosis agents. The pigments produced by atypical mycobacteria when grown in the light are "photochromogens". Some atypical mycobacteria produce pigment when grown in the dark - "scotochromogens". Others do not form pigment under any circumstances. They can frequently be found in domestic water supplies, and in the past have given rise to false positive results for the Ziehl Neelsen stain if the stain has not been made with filtered water.

Question 83

**Life-threatening Infections** Some infections are severe and have high mortality, even when treated appropriately. In these conditions, treatment must be instituted as soon as possible after the collection of appropriate specimens for diagnosis. Diagnostic medical microbiology laboratories devote considerable time and energy to these infections. You are provided with brief case histories, and questions pertinent to these cases. **Septicaemia** **following surgery** DB is a sixty-five-year-old man who was admitted to hospital for an elective transurethral resection to correct prostatic enlargement. Following the removal of his urinary catheter, the patient complained of nausea and chills. He developed a pyrexia and became prostrated. His pulse rate rose to 120 beats per minute and his breathing became rapid. A blood culture was taken and this yielded the Isolate 'A'. The bacterium is a motile, Gram-negative and oxidase-negative. On growth on MacConkey agar, it produces red colonies, indicating that it is a lactose fermenter. What is the identity of this bacterium?

Answer 83

- Escherichia coli; All of the bacteria in the list of options with which you have just been presented belong to the family Enterobacteriaceae. The bacterium responsible for the clinical symptoms, in this case, is Escherichia coli. Klebsiella pneumoniae is a non-motile lactose fermenter. Shigella sonnei is a “late” lactose fermenter and is non-motile. On first isolation, Shigella sonnei appears to be unable to ferment lactose and on MacConkey agar grows with buff-colored colonies typical of non-lactose fermenters but after prolonged incubation, it does ferment lactose, with its colonies taking on a red hue. Both Salmonella enterica var. Typhimurium and Yersinia pestis are non-lactose fermenters. The former causes gastroenteritis: the latter is responsible for the plague.

Question 84

"Chinese restaurant syndrome" was once associated with:

Answer 84

- Bacillus cereus "Chinese restaurant syndrome" is an intoxication caused by Bacillus cereus. E. coli O157 causes hemolytic uremic syndrome. Enteric fever is caused by Salmonella Typhi. Another name for dysentery, caused by Shigella dysenteriae is the "bloody flux". "Rice-water" stools are characteristic of cholera, caused by Vibrio cholerae.

Question 85

Mycoplasmas:

Answer 85

- are often present as commensals. - can be grown on laboratory media. - are resistant to penicillin. - are susceptible to erythromycin. Infections caused by mycoplasmas are usually treated with erythromycin or tetracyclines which act by inhibiting bacterial protein synthesis. These bacteria are resistant to penicillins because they lack a cell wall containing peptidoglycan, the target for this class of antibiotic. When growing on laboratory media, mycoplasmas produce very small colonies that have a characteristic "fried egg" appearance.

Question 86

**Post-surgical gangrene** DM, a diabetic aged 83, had a mid-thigh amputation following ischemia of a lower limb. Following the operation, the patient started to vomit, was distressed, pyrexial and confused. His pyrexia continued and he developed a tachycardia (110 beats per minute). Hematological examination showed a marked leukocytosis (2.4 x 109 cells per liter). Three days after the operation, the surgical wound showed evidence of breaking down. Extensive necrosis developed accompanied by marked edema. The wound was highly malodorous. You are provided with a Gram film prepared from a wound swab obtained from this patient. Are more than one species of bacterium present?

Answer 86

- Yes. Although the cause of gas gangrene is a sporing bacterium, it is rare to see spores in microscopic preparations from clinical specimens or from routine artificial cultures of this bacterium, a large, Gram-positive rod. Also presenting this preparation, stained red with the counterstain, are leukocytes that are degenerating. This damage is evidence of the powerful toxins that this pathogen produces. Other bacteria can be seen in this preparation, particularly Gram-negative rods. This is typical of gangrenous infections.

Question 87

The following may be found as part of the commensal microbiota of the skin:

Answer 87

- Staphylococcus saprophyticus. - Propionibacterium acnes. - Corynebacterium spp. The skin is dry and inhospitable. The commensals of skin are typically Gram-positive so the obligately anaerobic Gram-negative Bacteroides spp would only be isolated sporadically and are not part of the resident microbiota of the skin. Although they are Gram-positive, lactobacilli are not routinely isolated from normal, healthy skin. The microaerophilic Propionibacterium acnes is isolated as part of the resident skin microbiota, as is Staphylococcus epidermidis, although Staphylococcus epidermidis is the most common of the coagulase-negative staphylococci to be isolated from normal, healthy skin.

Question 88

What incubation conditions would you use to culture this (non-specific urethritis, tetanus, typhoid) bacterium?

Answer 88

- in anaerobic culture. The causative pathogen in this case is an obligate anaerobe. Oxygen must be excluded from any artificial culture if it is to be successful.

Question 89

**Gram** **film prepared from the smaller red colony isolated from the post-menopausal vaginal swab** It is a Gram-positive coccus that tends to form chains. Subsequent testing shows that this bacterium is not capable of releasing oxygen bubbles from a dilute solution of hydrogen peroxide, indicating that it is **catalase negative.** This isolate grows on MacConkey agar producing acid from the fermentation of lactose. It is thus considered to be a lactose fermenter. When this bacterium is plated onto fresh blood agar it does not alter the appearance of the blood cells. The identity of this bacterium is:

Answer 89

- Enterococcus faecalis. The vaginal microbiota changes dramatically through life. When a baby girl is born, she still carries her mother's hormones for a short while, and the vaginal microbiota is dominated by lactobacilli, sometimes called "Döderlein" bacilli. As the maternal hormones decline, the microbiota changes to resemble that of the bowel. However, with the onset of puberty reproductive hormones are produced, and the lactobacilli return. At the menopause, the microbiota reverts to resemble that of the bowel once more. Upon aerobic culture, E. coli and enterococci are the predominant species.

Question 90

Should further cases of meningitis be expected from children attending the same school?

Answer 90

- Yes. Although further cases of meningitis may occur in this school, they are only likely to be among close contacts of JC who refuse prophylaxis.

Question 91

In the collection of urine samples for laboratory analysis:

Answer 91

- catheter specimens are easily contaminated. - a mid-stream specimen is used to wash out bacteria from the distal urethra that would otherwise contaminate the specimen. Collection of urine samples that are of use to the diagnostic laboratory is more difficult than one may think since they are easily contaminated and may suffer overgrowth of bacteria if there is a delay from collection to specimen analysis. Mid-stream specimens are used; in these microorganisms from the distal urethra are flushed out before specimen collection begins. The external genitalia should be washed carefully before collecting urine samples but with soap and water rather than with a disinfectant, which may contaminate the specimen, making any bacteria. present impossible to recover in artificial culture. Catheter specimens are notoriously easy to contaminate, as are 'bag' specimens, which are regarded by Microbiologists as nothing more than a waste of time. Urine is an excellent growth medium and to prevent overgrowth in a clinical specimen, the bacteriostatic agent boric acid (not formic acid) is added to stop this from happening.

Question 92

Campylobacter jejuni:

Answer 92

- is the commonest cause of bacterial gastroenteritis in the UK. - may be acquired by consumption of unpasteurized milk. - is microaerophilic. - may be isolated from a diversity of bird species. Campylobacter jejuni is a microaerophilic, curved Gram-negative bacterium that is the commonest cause of bacterial intestinal infection in the UK. Although the vector for infection may be difficult to identify, the infection may follow the consumption of unpasteurized milk. It may be isolated from a wide diversity of bird species.

Question 93

The following bacteria have may be isolated from animals other than humans:

Answer 93

- Campylobacter jejuni. - Salmonella Typhimurium. Campylobacters and many salmonellas other than Salmonella Typhi may be isolated from a range of mammals and birds; some salmonella serovars may be isolated from reptiles. Salmonella Typhi, Corynebacterium diphtheriae and Vibrio cholerae are uniquely human pathogens.

Question 94

The following bacteria have humans as their only reservoir of infection:

Answer 94

- Salmonella Typhi. - Vibrio cholerae - Corynebacterium diphtheriae. Salmonella Typhi, Corynebacterium diphtheriae and Vibrio cholerae are uniquely human pathogens. Campylobacters and many salmonellas other than Salmonella Typhi may be isolated from a range of mammals and birds; some salmonella serovars may be isolated from reptiles.

Question 95

The following may be found as part of the commensal microbiota of the colon:

Answer 95

- Bacteroides spp. - Enterococcus faecium. - Escherichia coli.

Question 96

The following may be associated with the production of stones in the urinary tract:

Answer 96

- Proteus mirabilis. Proteus mirabilis breaks down urea to release ammonia. This raises the pH of urine, which, in turn, causes the precipitation of inorganic salts, particularly leading to the formation of phosphate crystals. This may lead to the production of amorphous calcium phosphate or magnesium phosphate crystals that resemble coffin lids when viewed microscopically. These crystals act as the foci for stone formation.

Question 97

The following processes are common to all bacterial pathogens:

Answer 97

- they multiply in the host. - they are shed from the host. - they enter the host and interact with host tissue. All bacterial pathogens enter the host and bind to host tissue; they multiply within the host and are eventually shed from the host. However, only intracellular pathogens enter host cells. Also, although many pathogens invade at mucosal surfaces, this is not the case for all e.g. skin infection.

Question 98

The following cause(s) ulceration of the gut sub-mucosa leading to the "bloody flux":

Answer 98

- Shigella dysenteriae. Shigella dysenteriae causes dysentery, sometimes known as the "bloody flux". Bleeding results from ulceration of the gut sub-mucosa.

Question 99

**The Human Commensal Microbiota in Health and Disease** There is a variety of microorganisms that interact with humans, exploiting a wealth of microenvironments. Certain parts of the body are sterile. These include solid organs, blood, cerebrospinal fluid, and urine. The lower respiratory tract in health is also sterile. This sterility in an open system is due to the action of the 'mucociliary escalator' that sweeps foreign matter back out of the lungs. Exposed sites are colonized by large numbers of microorganism that are highly adapted to their particular environment. These organisms constitute the commensal microbiota of the body. In diagnostic microbiology, it is important to distinguish between a patient's commensal microbiota and the causative agent of the infection from which they are suffering. This is not always an easy process, since in some cases microbes are normal microbiota at certain anatomical locations, but are considered pathogens when isolated from other sites. Most of the microbes discussed are bacteria, although some fungi and certain protists also contribute to the human commensal microbiota. Viruses are a special case. There is still debate concerning the nature of viruses, certainly, they are structures that are incapable of autonomous existence. In many cases virus infection is entirely to the detriment of the host. However, certain viruses have the ability to form Latent Infections. A good example of a virus that causes a latent infection is the herpesvirusherpes simplex 1 or HSV 1. This virus infects the vast majority of humans sometime during early childhood, and often the primary infection is inapparent. It infects the mucous membranes around the mouth, and the virus spreads to the trigeminal ganglion. There it enters a latent phase. The virus DNA persists, but there is little or no production of novel virus particles. When the host is subjected to stresses of various kinds, the latent virus can undergo a reactivation. New virus particles are produced, and these spread back to the mucous membranes around the mouth, where they erupt to cause the characteristic 'cold sores', suffered by a number of people. Certain individuals are more prone to developing cold sores than others, but the majority of people World wide carry HSV 1 as a latent infection. Because it is so widespread, and because, at least for the majority of the time it causes so little problem, it has been argued that HSV 1 and other latent viruses may be considered as commensal with humans. On the following page is a diagram of the human body. From the results of your identification tests, and with reference to any books that you may wish to use, you should annotate this picture, marking the commensal microbiota that you would expect to find at the sites marked. **Culture** **of a skin swab growing on fresh blood agar** The culture illustrated was made by streaking a skin swab onto fresh blood agar and incubating aerobically overnight. Two colony types are apparent, a smallish, whitish colony and a flat, dry-looking grey colony. **Gram** **film prepared from the skin swab** In the Gram film, two different types of cell can be seen; one is rod-shaped (a bacillus) and the other is round (a coccus). Both are Gram-positive. Thus, this film contains both Gram-positive bacilli and cocci. The Gram-positive cocci are said to resemble bunches of grapes. Some bacteriologists say that the Gram-positive bacilli in this film resemble 'Chinese letters'. **Gram** **film prepared from the small, white colony isolated from the skin swab** This bacterium is a Gram-positive coccus. Subsequent testing shows that this bacterium is capable of releasing oxygen bubbles from a dilute solution of hydrogen peroxide, indicating that it is catalase positive. This isolate cannot cause clot formation when suspended in plasma. The identity of this bacterium is:

Answer 99

- a coagulase-negative staphylococcus.

Question 100

Lobar pneumonia:

Answer 100

- may be complicated by meningitis. - may be caused by commensal mouth and nose organisms. Pneumococcal meningitis is a rare complication of lobar pneumonia. For a definitive diagnosis, isolation of Streptococcus pneumoniae from blood cultures is required. Generally, three sets are taken to maximize the chances of obtaining a positive culture, since the presence of bacteria in the blood is intermittent. Pneumonia caused by Pneumocystis jiroveci is seen in patients who are immunocompromised, but it is not confined to a particular lobe.

Question 101

In pulmonary tuberculosis:

Answer 101

- combinations of anti-tuberculous drugs are used to avoid the selection of drug-resistant mutants during therapy. - reactivation of primary disease involves mostly the apical or posterior segments of the upper lobes. - serological tests are rarely of use in diagnosis. - infection may be more severe in patients with AIDS. Because of the length of time that treatment extends over, the risk of developing resistant strains is considerable when a single anti-mycobacterial is used. This risk is reduced considerably by employing combination therapy. The tuberculin reaction (Mantoux test) may be negative in, among others, the elderly, patients receiving corticosteroids, and also in patients with overwhelming tuberculosis. Reactivation most often affects the apical or posterior segments of the upper lobes of the lung reputedly because the concentration oxygen level is greater at these sites. Individuals with AIDS are also susceptible to infection with atypical mycobacteria especially Mycobacterium avium intracellulare.

Question 102

**Endogenous infections** Gram-positive bacteria contain large quantities of peptidoglycan. This is a polymer that confers considerable mechanical strength to the wall, enabling bacteria to withstand considerable pressures, including osmotic pressures. In contrast, Gram-negative bacteria contain very little peptidoglycan in their cell walls. What is the Gram reaction of the majority of bacteria on the outer surface of the body?

Answer 102

- Gram-positive

Question 103

Pair up the pathogens with the infection that each is most likely to cause

Answer 103

* Escherichia coli - urinary tract infection in a young woman. * Staphylococcus aureus - a boil in an adolescent boy. * Bacteroides fragilis - a wound abscess following colonic surgery. * a "viridans" streptococcus - endocarditis developing after extensive dental work. * a coagulase-negative staphylococcus - endocarditis following heart valve replacement. Bacteroides fragilis is a commensal found in very large numbers in the bowel. As such, it is a frequent contaminant of wounds following colonic surgery. Escherichia coli is another commensal found in the bowel. It is the commonest cause of urinary tract infections in women. In these cases, infection typically originates from bacteria in the bowel microbiota. In women who have recently become sexually active, Staphylococcus saprophyticus, a skin commensal, is a common isolate from cases of "honeymoon cystitis". This bacterium was not on your list of choices. Boils, the posh name for which is "furuncles", are almost always caused by Staphylococcus aureus. Endocarditis may be caused by many bacteria. Indeed, Staphylococcus aureus is often isolated from people who inject drugs intravenously and who develop endocarditis. In the two cases in this exercise, Staphylococcus epidermidis, a skin commensal, is frequently isolated from cases following valve replacement. This bacterium is derived from the skin. "Viridans" streptococci are found in large numbers in the oral microbiota and can be isolated from cases of endocarditis when the patient has undergone extensive dental work and who have heart valves that have previously suffered some damage. The role of dentistry is, however, now being played down in cases of endocarditis. People with poor oral hygiene are also likely to suffer endocarditis caused by viridans streptococci if their heart valves are in some way compromised.

Question 104

What is Haemophilus influenzae normal habitat?

Answer 104

- the upper respiratory tract. Haemophilus influenzae may typically be isolated from the upper respiratory tract. In cases of meningitis, it gains access to the meninges via the bloodstream – the **hematogenous** **route**. Examples of other infections caused by Haemophilus influenzae include acute epiglottitis, middle ear infections, exacerbation of bronchitis... Pathogenic strains all possess a capsule – hence the Pittman typing scheme. This is implicated in the disease process because it helps to prevent phagocytosis and once phagocytosis has occurred it protects the bacterium against intracellular killing. Examples of other infections caused by Haemophilus influenzae include acute epiglottitis, middle ear infections, exacerbation of bronchitis... Pathogenic strains all possess a capsule – hence the Pittman typing scheme. This is implicated in the disease process because it helps to prevent phagocytosis and once phagocytosis has occurred it protects the bacterium against intracellular killing.

Question 105

Enteric fever is caused by:

Answer 105

- Salmonella Typhi.

Question 106

The following antibiotics may be used in treating community-acquired pneumonia:

Answer 106

* cephradine * penicillin * erythromycin * ampicillin Fusidic acid is used in the treatment of staphylococcal infections and, since mutations to resistance occur at a relatively high frequency, this agent is not used alone.

Question 107

The following antibiotics have been shown to be of value in treating patients with Legionnaire's disease:

Answer 107

* rifampicin * erythromycin The β-lactams, including ampicillin, cefotaxime, and cephradine are not used to treat Legionnaire's disease because Legionella pneumophila produces β-lactamase, but all the other agents listed may be used to treat this condition.

Question 108

From what other infections are IV drug users at risk?

Answer 108

* hepatitis B infection; * hepatitis C infection; * human immunodeficiency virus infection. She is at risk from hepatitis B, hepatitis C and HIV. She may also be at risk from as yet undescribed viruses that are transferred by contact with blood and/or mucous membranes. Hepatitis A is not transmitted by the blood-borne route.

Question 109

Diphtheria:

Answer 109

* has recently caused epidemics in Eastern Europe. * virtually never follows vaccination. * is caused by bacteria resembling organisms found in the normal skin. * the vaccine is a denatured-toxin (toxoid). Although diphtheria is now rare in the United Kingdom, it has recently caused epidemics in countries in Eastern Europe. The low incidence of diphtheria in the United Kingdom is due to the success of the vaccination programme, where the diphtheria vaccine is usually given together with pertussis and tetanus vaccines (DPT). It is the measles vaccine that is given with the mumps and rubella vaccines.

Question 110

The following may be found as part of the commensal microbiota of the conjunctivae:

Answer 110

- Corynebacterium spp. Being bathed in tears that are salty and contain antimicrobial substances such as lysozyme, the eye is a particularly inhospitable environment for most bacteria. Nevertheless, corynebacteria may be isolated in small numbers from the conjunctivae.

Question 111

The following pathogens may be associated with arthritis:

Answer 111

* Staphylococcus aureus * Salmonella enterica * Neisseria gonorrhoeae * rubella virus The commonest cause of septic arthritis is Staphylococcus aureus. Septic arthritis is a rare complication of gonococcal infection. Salmonella enterica is an occasional cause of septic arthritis, particularly in children. Rubella virus may cause reactive arthritis. Herpes simplex virus is not associated with arthritis.

Question 112

The following may be a common cause(s) of urinary catheter associated infection:

Answer 112

* Proteus mirabilis. * Escherichia coli. Staphylococcus saprophyticus causes acute lower urinary tract infection in sexually active women. Entamoeba histolytica and Cryptosporidium parvum are protists that cause diarrhoeal illness. Escherichia coli and Proteus mirabilis are well-recognized causes of colonization and infection of the catheterized urinary tract.

Question 113

The following bacteria may be isolated as part of the commensal microbiota of the lungs:

Answer 113

- none of the above The lungs are kept sterile by the action of the mucociliary escalator.

Question 114

The following infections show a sex bias:

Answer 114

* cystitis * leptospirosis * streptococcal skin infection. Neither tuberculosis nor typhoid shows a sex bias. Cystitis is more common in women than men, although this bias lessens with age and infant boys are more susceptible to urinary tract infections than are young girls. Boys are much more likely than girls to suffer from streptococcal skin infections because they are much more likely to suffer skin abrasions, allowing a portal of entry for Streptococcus pyogenes. Leptospirosis, also known as Weil's disease, is more common in men than in women, because of the increased risk of occupational exposure to infected rat urine, the vector for this infection.

Question 115

Endotoxin is:

Answer 115

* produced by Gram-negative bacteria. * functional through a variety of mechanisms that damage the host. * a component of the bacterial cell envelope. Endotoxin is a structural feature, the lipid A component of lipopolysaccharide, found in the outer leaflet of the outer membrane of Gram-negative bacteria, released on the disruption of the bacterial cell. Endotoxin is cytotoxic to a wide range of host cells and over-stimulates the host defences to produce symptoms of shock - fever with a high pulse rate, rapid breathing rate and low blood pressure.

Question 116

Endocarditis in an IV drug user BS is 24 years old and has been using intravenous drugs for several years. She was brought into her local Casualty Department by friends who said that she had appeared well until recently. She had a high temperature and was delirious when examined by the Casualty Officer, who noted that B had at least one heart murmur. Three sets of blood cultures were taken whilst B was in Casualty, and she was then promptly admitted as an in-patient. The blood cultures yielded Isolate ‘B’; a Gram-positive catalase-positive coccus that causes the plasma to clot. What is the identity of this bacterium?

Answer 116

- Staphylococcus aureus. The bacterium responsible for BS’s symptoms is Staphylococcus aureus. Many common bacteria can cause endocarditis; often in people with fragile health. In most cases, endocarditis is a disease that progresses slowly. Staphylococcus aureus, however, is an aggressive pathogen and patients very rapidly become ill and, without treatment, will die. Indeed, without antibiotic therapy that kills the target pathogen, all cases of endocarditis are fatal, irrespective of the causative bacterium. Regarding endocarditis, Staphylococcus aureus is an unusual pathogen in another respect. Most cases of endocarditis affect the valves on the left of the heart. These are larger than the valves on the right of the heart and so are more likely to be targeted by pathogens. The most commonly affected valve is the mitral valve, then the aortic valve, followed by the tricuspid valve and finally the pneumonic valve. In intravenous drug users, Staphylococcus aureus is introduced into a vein during injection. This will transport the bacterium to the right side of the heart, where it typically infects the tricuspid valve.

Question 117

Organisms associated with atypical pneumonia include:

Answer 117

* influenzae viruses. * Chlamydia psittaci * Chlamydia pneumoniae Haemophilus influenzae is associated with exacerbations of chronic bronchitis. Chlamydia psittaci is associated with pneumonia that particularly affects bird fanciers, especially those who keep members of the parrot family. These are referred to as psittacine birds. Mycoplasma pneumoniae is a common cause of atypical pneumonia, not Mycobacterium hominis.

Question 118

The following may be part of the commensal microbiota of the vagina of a woman of reproductive age:

Answer 118

- Lactobacillus spp. Lactobacilli dominate the commensal microbiota of the vagina in women of reproductive age. The commensal microbiota of a post-menopausal vagina resembles the fecal microbiota. Staphylococcus saprophyticus is a skin commensal that causes "honeymoon cystitis", an infection of the urinary tract rather than the genital tract.

Question 119

Among the causes of cellulitis is/are:

Answer 119

* Streptococcus pyogenes. * Pseudomonas aeruginosa. * Staphylococcus aureus. The commonest cause of cellulitis is Streptococcus pyogenes but the condition may rarely be caused by Staphylococcus aureus. Immunocompromised individuals may suffer cellulitis caused by Pseudomonas aeruginosa, which may develop into gangrene. As its name suggests, Mycobacterium leprae causes leprosy; Mycobacterium ulcerans is the cause of scrofula.

Question 120

Endocarditis following valve replacement DV, a retired long-distance lorry driver, had a long-standing aortic valvular disease of such severity that it warranted replacement with a prosthetic valve. Eight months postoperatively, he began to feel vaguely unwell. At a routine outpatient appointment, he was seen by his cardiac surgeon, who decided to admit him for further tests. These included blood cultures, which yielded a bacterium, Isolate ‘C’, similar to that grown from BS's specimen. This bacterium is a Gram-positive, catalase-positive coccus that is unable to cause clot formation when suspended in plasma, whereas the isolate from BS was able to clot plasma. What is the likely identity of this isolate?

Answer 120

- a coagulase-negative staphylococcus; In this case, the causative bacterium is a coagulase-negative staphylococcus. Because this is a common skin contaminant for blood cultures, this isolate will have been identified more fully than is usual. The most likely identity in this case will be Staphylococcus epidermidis. In identifying endocarditis caused by coagulase-negative staphylococci, care must be taken not to misidentify a skin contaminant. In this case, this is particularly important since the symptoms started eight months after the valve replacement that predisposed this patient to his endocarditis. Because of this delay, the case is described as being of late onset. To ensure that the coagulase-negative Staphylococcus isolated in this case is a pathogen responsible for endocarditis rather than a skin contaminant, multiple blood cultures will have been taken. Whenever possible, these will have been taken from different sites. The bacterium will need to have been recovered in more than one culture and these isolates will have been indistinguishable on typing.

Question 121

Acute bacterial otitis media in children:

Answer 121

- often resolves spontaneously without antibiotics. Acute otitis media is generally unilateral in its presentation, and the condition often resolves without the need for antibiotics. Otitis media does not display a sex bias and, if the eardrum ruptures, the resultant hearing loss is not permanent. One possible cause of otitis media is Haemophilus influenzae, but it is associated with a variety of bacteria and viruses.

Question 122

Pneumocystis jiroveci:

Answer 122

* colonizes many healthy adults asymptomatically * is classified as a fungus Studies of the ribosomal RNA structure of this organism has placed its classification with the fungi, even though its microscopic appearance and general behaviour would tend to place it together with the Protista. It can be found colonising many healthy individuals, and antibodies are frequently found in normal persons. Pneumocystis jiroveci pneumonia is an AIDS-defining infection, but disease caused by this organism was first described in severely malnourished children.

Question 123

**Culture** **of a nasal swab growing on fresh blood agar** The culture illustrated was made by streaking a nasal swab onto fresh blood agar and incubating aerobically overnight. Two colony types are apparent, a smallish, golden colored colony and a flat, dry-looking grey colony. **Gram** **film prepared from the nasal swab** In the Gram film, two different types of cell can be seen; one is rod-shaped (a bacillus) and the other is round (a coccus). Both are Gram-positive. Thus, this film contains both Gram-positive bacilli and cocci. The Gram-positive cocci are said to resemble bunches of grapes. Some bacteriologists say that the Gram-positive bacilli in this film resemble 'Chinese letters'. **Gram** **film prepared from the golden colony isolated from the nasal swab** This bacterium is a Gram-positive coccus. Subsequent testing shows that this bacterium is capable of releasing oxygen bubbles from a dilute solution of hydrogen peroxide, indicating that it is **catalase positive**. This isolate causes clot formation when suspended in plasma. The identity of this bacterium is:

Answer 123

- Staphylococcus aureus

Question 124

The following produce spores that can be readily observed microscopically:

Answer 124

* Clostridium tetani * Clostridium botulinum * Bacillus anthracis

Question 125

Certain fungal species or structures may be described as:

Answer 125

* psychrophiles * mesophiles * geotropic * thermophiles Fungi are not photosynthetic, requiring a supply of organic carbon for growth. Most fungi grow at moderate temperatures as mesophiles, growing at temperatures between 10 and 40°C, with optima between 25 and 35°C, and this group includes fungal pathogens of mammals. Fungal psychrophiles grow at temperatures below 20°, and sometimes below 0°C. They may contaminate meat stored in refrigerators. Thermophilic fungi are generally considered to grow at temperatures ranging from 20 to 50°C and with optima at or near 40°C and maximum growth temperature of between 60 and 62°C. They play an important role during composting of plant waste material. Reproductive structures such as basidiocarps and sporangiophores may exhibit positive photropism - growth towards the light, and/or negative geotropism - growth away from the center of the earth due to the influence of the force of gravity. Do not confuse PHOTOTROPISM - the movement towards the light with PHOTOTROPISM - using light as a source of metabolic energy.

Question 126

A smear from the centrifuged deposit of the cerebrospinal fluid (CSF) from a child of 5 months with meningitis showed Gram-negative pleomorphic bacilli. The bacteria likely to be incriminated include:

Answer 126

* Haemophilus influenzae Lactobacilli do not cause meningitis and are Gram-positive rods. Listeria monocytogenes causes meningitis but again the organism is a Gram positive rod. Streptococcus pneumoniae are Gram-positive cocci usually in pairs and, although Neisseria meningitidis is Gram-negative, the organism is a coccus.

Question 127

Clostridium difficile:

Answer 127

* is associated with the use of broad-spectrum antibiotics. Although typically associated with the use of broad-spectrum antibiotics, the use of every class of antimicrobial has been linked with the occurrence of diarrhea caused by Clostridium difficle. While most reports of C. difficile infection emanate from hospitals, cases of infection in the wider community do occur. Use of antibiotics suppresses the commensal microbiota in the gut and this permits overgrowth of Clostridium difficle rather than the pathogen-suppressing the microbiota itself. It is Clostridium perfringens that is associated with gastrointestinal illness in institutions, particularly where meat dishes are re-heated. Endotoxin is associated with Gram-negative bacteria; Clostridium spp. are Gram-positive.

Question 128

The following factors are important in the pathogenesis of infective endocarditis:

Answer 128

* the ability of viridans streptococci to produce dextran. * platelets Dextran production on the surface of viridans streptococci appears to promote adhesion to damaged areas of valvular endothelium coated with fibrin and platelets. The host response to infection is very limited, due to sequestration of the organisms within the valve vegetations. Endotoxin plays no significant role in this disease.

Question 129

Targets for antibacterial agents include:

Answer 129

* protein synthesis. * metabolic activities. * the integrity of cell membranes. * cell wall synthesis. * nucleic acid synthesis. All of these processes are targets of antibacterials. Examples of each mechanism of action are given below: the β-lactam antibiotics including penicillins, cephalosporins, monobactams, and carbapenems act against cell walls. Protein synthesis may be inhibited by many agents, including aminoglycosides, macrolides, chloramphenicol, and the tetracyclines. Nucleic acids are the targets for the quinolones and the integrity of cell membranes may be disrupted by colistin and polymyxins. Trimethoprim and sulphonamides interface with bacterial metabolism.

Question 130

Peptidoglycan is:

Answer 130

* a component of the bacterial cell wall. * found in both Gram-positive and Gram-negative cell walls. * responsible for maintaining the shape of many bacteria. Peptidoglycan is only found in the cell walls of bacteria and never in fungi or other eukaryotic cells. It is a rigid and very strong polymer that lends the bacterial cell its shape.

Question 131

In categorising bacteria as Gram-negative or positive:

Answer 131

* the reaction is independent of cell shape. * there is an association between the stain reaction and the susceptibility to certain antibiotics. * the cell envelope composition determines the reaction. Gram-positive cell walls contain more peptidoglycan than Gram-negative bacteria, and the Gram-negative cell is surrounded by an additional outer membrane. Bacteria exhibiting either Gram reaction can be round (cocci) or rod-shaped (bacilli). It is not the cell wall that is stained with the Gram-stain, but its structure determines whether or not the cell can retain the crystal violet-iodine complex. Because of the fundamental differences in the cell structure, there is an association with the Gram reaction and susceptibility to certain antibiotics. The Gram protocol requires heat fixed bacteria and exposure to organic solvents. This treatment kills all bacteria, some of which appear Gram-negative upon subsequent Gram staining.

Question 132

Viroids are:

Answer 132

* small, circular single stranded RNA molecules.

Question 133

Antibacterials inhibiting bacterial cell wall synthesis include:

Answer 133

* penicillins. * cephalosporins Penicillins and cephalosporins inhibit bacterial cell wall synthesis, aminoglycosides and chloramphenicol inhibit bacterial protein synthesis and quinolones interfere with DNA packaging.

Question 134

Fungi:

Answer 134

* are eukaryotes. * can be divided into yeasts and molds. * are mostly saprophytes (living on dead matter). The cells of fungi have membrane-bound nuclei and organelles and so fungi are eukaryotic. Yeasts are unicellular fungi that multiply by a process of "budding": molds are filamentous fungi. Many fungi are microscopic, but molds can grow to be very large, as with bracket fungi, mushrooms, toadstools, and puffballs. Fungi have cell walls of a different structure from the cell walls of either bacteria or plants. Many fungi live on dead organic matter and are thus saprophytes.

Question 135

Yeasts:

Answer 135

* are predominantly unicellular. * may have polysaccharide capsules. * may reproduce by budding. Yeast is unicellular fungi that typically reproduce by budding. They do not produce air-borne spores. Cryptococcus neoformansproduces a polysaccharide capsule that protects it when it causes meningitis. Peptidoglycan is a uniquely bacterial polymer.

Question 136

A non-blanching skin rash commonly accompanies meningitis caused by:

Answer 136

* Neisseria meningitidis Neisseria meningitidis frequently causes septicemia as well as meningitis, one of the symptoms of which is a non-blanching skin rash, as shown by the 'glass' test. This is due to leakage of blood from the small blood vessels and is one of the symptoms of the severe endotoxic shock caused by this bacterium.

Question 137

The Ziehl-Neelsen (ZN) stain is used:

Answer 137

* to visualise Mycobacterium tuberculosis. * in the presumptive diagnosis of tuberculosis. * to visualise Mycobacterium leprae. Mycobacterium tuberculosis, the causative agent of tuberculosis, and Mycobacterium leprae, the cause of leprosy, are among a limited number of pathogens that are difficult to stain and are acid- alcohol-fast by stains such as the Ziehl Neelsen stain. This stain is now often replaced by the rhodamine/auramine fluorescent stain.

Question 138

Peptidoglycan contains:

Answer 138

* N-acetylglucosamine * N-acetylmuramic acid * amino acids Peptidoglycan is a polymer component of the bacterial cell wall and comprises a backbone of alternating N-acetylmuramic acid and N-acetylglucosamine which is cross-linked by peptide chains composed of amino acids. It is a unique biopolymer, containing both D- and L-amino acids.

Question 139

A bacterium grows best at 42°C. It cannot grow in air but requires ~2% oxygen. It may be accurately described as:

Answer 139

* a microaerophile. A growth optimum of 42°C falls within the mesophilic temperature range, although because this is higher than the core human body temperature of 37°C, pathogenic bacteria with this growth optimum are frequently described as thermophilic. Its requirement for a low concentration of oxygen makes this bacterium a microaerophile.

Question 140

The lipopolysaccharide of the outer membrane of Gram-negative bacteria can be toxic and is known as:

Answer 140

- LPS - endotoxin A toxoid is a toxin that has lost its biological activity, but that retains its antigenic structure, stimulating the production of antibodies that can protect against the activity of the native toxin. The genus Toxoplama includes Toxoplasma gondi, a protist parasite. Exotoxins are proteins secreted by bacteria, and harmful to higher organisms. The abbreviation often used for lipopolysaccharide is LPS. This can cause the symptoms of Gram-negative shock, including elevated body temperature, low blood pressure, and rapid heartbeat, and it is sometimes referred to as endotoxin as a consequence.

Question 141

Increased vulnerability of hospital patients to infection may be due to:

Answer 141

- lying in bed. - presence of an indwelling urinary catheter. - a surgical wound. - the need for artificial ventilation to assist respiration. - presence of an intravenous line.

Question 142

The following are examples of "prion" diseases:

Answer 142

- Creutzfeldt-Jakob disease - kuru. Guillain-Barré syndrome is an acute disease caused by an autoimmune reaction that affects the peripheral nervous system. It is usually triggered following an infection. Its symptoms include weakness and paralysis. Burkitt's lymphoma is associated with Epstein-Barr virus infection, as is nasopharyngeal carcinoma. Subacute sclerosing panencephalitis is a very rare, fatal complication of measles. "Prion" diseases are thought to be due to a self-replicating protein agent(s), lacking DNA or RNA. Pathologically they present as slowly-developing but fatal spongiform encephalopathy.

Question 143

The following statements are true of bacteraemia:

Answer 143

- it may be diagnosed by blood culture. - it may be associated with the insertion of medical devices into the bloodstream. - it is likely to occur during dental procedures. Bacteraemia due to viridans streptococci is a common complication of dental procedures, but it is often asymptomatic. Nosocomial cases may represent infection associated with the insertion of medical devices into the bloodstream, since these breach normal anatomical barriers to infection. Bacteraemia is diagnosed by blood culture.

Question 144

Antibacterials acting upon DNA packaging include:

Answer 144

- quinolones Quinolones act on DNA packaging, penicillins and cephalosporins act on cell wall synthesis and bacterial protein synthesis is inhibited by aminoglycosides and chloramphenicol.

Question 145

Flower vases are no longer found on the wards of NHS hospitals. The water was a reservoir for the following nosocomial pathogens:

Answer 145

- Acinetobacter spp. - Pseudomonas aeruginosa Gram-negative bacteria such as Acinetobacter sp. and Pseudomonas aeruginosa are frequently isolated from the water in flower vases.

Question 146

Antimicrobial agents widely used for the treatment of anaerobic infections include:

Answer 146

- metronidazole Aminoglycosides, such as amikacin, gentamicin, and neomycin have little activity against anaerobes. Indeed, neomycin can be added to artificial growth media to select for obligate anaerobes. Although penicillin G can be active against some anaerobes, others produce β-lactamases: enzymes capable of inactivating penicillins. Metronidazole is widely used to treat anaerobic bacteria and is also active against flagellate protists.

Question 147

Among the bacteria having cell walls from which peptidoglycan is absent are:

Answer 147

- mycoplasmas - chlamydia Although spirochaetes are flexible, they have peptidoglycan cell walls of Gram-negative structure. Staphylococci and clostridia are both Gram-positive and have cell envelopes that include many layers of peptidoglycan. Chlamydia and mycoplasmas use other means to maintain cell integrity or rigidity.

Question 148

The bacterial flagellum:

Answer 148

- is a rigid helical structure, producing motility by rotation. - is composed of a protein called flagellin. Bacterial flagella (singular - flagellum) are unlike the more complex eukaryotic cilia and flagella, which may have wave-like or whip-like activity. Flagella propel bacteria by rotatory motion. They are composed of protein, and act as antigenic structures. Not all bacteria produce flagella, not even all motile bacteria. Bacteria without flagella may demonstrate "gliding motility".

Question 149

Viruses are identified as a separate group of microorganisms because:

Answer 149

- they contain only a single type of nucleic acid in the virion. - of their mechanism of replication. Viruses are obligate intracellular parasites with a characteristically simple organization and mode of replication although the chlamydia is also obligate intracellular parasites. They do not possess cell walls or nuclei, and cannot thus be described as eukaryotic. Being akaryotic, they do not have a cellular structure and thus cannot be one of the prokaryotes. The term "akaryote" may be misleading - it simply means "lacking a nucleus". Thus, erythrocytes may be described as akaryotic. It is debatable whether viruses are even truly alive.

Question 150

The outer membrane of Gram-negative bacteria contains:

Answer 150

- lipopolysaccharides. - proteins. - phospholipids. The outer membrane, characteristic of Gram-negative cells, lies beyond the periplasm and this lies above the peptidoglycan layer. As is typical of biological membranes, it contains phospholipids and proteins, but its outer layer also carries lipopolysaccharide, the major somatic antigen of Gram-negative bacteria.

Question 151

Structures lying outside the bacterial cell wall include:

Answer 151

- pili - the Gram-negative outer membrane - fimbriae - a capsule Mesosomes are invaginations of the cell membrane and thus lie within the cell wall. All the other structures lie outside cell walls. Capsules help to protect bacteria from phagocytosis, and fimbriae help bacteria to adhere to surfaces. Pili are involved in bacterial gene transfer, and the Gram-negative outer membrane acts as endotoxin.

Question 152

Viruses are identified as a separate group of microorganisms because:

Answer 152

- of their mechanism of replication. Viruses are obligate intracellular parasites with a characteristically simple organization and mode of replication. There are, however, other organisms that are obligate intracellular parasites. They do not possess cell walls or nuclei and are thus described as akaryotes. The term "akaryote" may be misleading - it simply means "lacking a nucleus". Thus, erythrocytes may be describes as akaryoti

Question 153

Viruses are classified by:

Answer 153

- morphology - genomic nucleic acid Viruses are classified on their morphology, genomic material and the organization of their genome. Their hosts and diseases caused are not now considered relevant. A good example is the disease hepatitis where there are at least five completely different viruses that cause hepatitis in man. Each virus belongs to a different family.

Question 154

Septicaemia due to Escherichia coli is commonly associated with:

Answer 154

- endotoxic shock - acute pyelonephritis Escherichia coli is an important component of the aerobic bowel flora. It is commonly associated with ascending urinary tract infections, resulting in pyelonephritis with positive blood cultures. It can also escape directly from the bowel into the bloodstream in patients rendered neutropenic as a result of disease or chemotherapy. It is a well-recognized cause of meningitis, but in neonates and not in adults. The endotoxin associated with the outer leaflet of the outer membrane may lead to septic shock.

Question 155

The following bacterial species are commonly encountered as contaminants in blood cultures:

Answer 155

- Pseudomonas fluorescens - Staphylococcus epidermidis - Propionibacterium acnes All these species may be pathogenic. Staphylococcus epidermidis and Propionibacterium acnes are, however, more commonly found as skin contaminants, rather than as the cause of septicaemia, and Pseudomonas fluorescens is a well-recognised contaminant associated with citrate bottles used to collect blood samples for coagulation studies.

Question 156

Ignaz Semmelweis investigated:

Answer 156

- child bed fever - puerperal fever - iatrogenic infection Although Creutzfeldt Jakob disease may be transmitted iatrogenically (through the activity of doctors), it had not been recognized in Semmelweis's time. He was concerned with the cause of puerperal fever, also known as child bed fever, and discovered the vector was cadaveric material, transmitted on the hands of medical students who attended confinements soon after attending post mortem examinations and without washing their hands between engagements. Hand washing drastically reduced the incidence of puerperal fever, but Semmelweis was not believed by the medical establishment of the day.

Question 157

Treatment of infective endocarditis:

Answer 157

- requires intravenous antibiotic treatment for 2-4 weeks. Antibiotic therapy for 2-4 weeks is required using a bactericidal agent or combination administered intravenously. Viridans streptococcal infection can usually be successfully treated without recourse to valve replacement. Susceptibility testing should include determination of the Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) of the causative organism. Endocarditis in drug addicts is commonly due to Staphylococcus aureus. Because many strains produce a β-lactamase, benzylpenicillin (penicillin G) would not normally be used to treat infections due to this species of bacterium.

Question 158

Common causative organisms of acute bacterial meningitis in the adult are:

Answer 158

- Neisseria meningitidis - Streptococcus pneumoniae Staphylococcus aureus is a rare cause of meningitis following neurosurgical procedures, spread, as a result of septicemia or in diabetes. Enterococcus faecalis is an extremely rare cause of meningitis. Escherichia coli classically causes meningitis in neonates (\<28 days old) probably due to the lack of transport of maternal IgM.

Question 159

Benzylpenicillin is effective in the treatment of meningitis caused by:

Answer 159

- Neisseria meningitidis Escherichia coli and Salmonella enterica are intrinsically resistant to benzylpenicillin, as are all coliforms. Haemophilus influenzaetype b may produce a TEM-type β-lactamase active against antibiotics such as ampicillin and benzylpenicillin. Mycobacteria are always resistant to β-lactams. Isolates of N. meningitidis from the UK are \<99% likely to be sensitive to benzylpenicillin.

Question 160

The following statements are true concerning infective endocarditis:

Answer 160

- the valves on the right side of the heart are most commonly affected in intravenous drug users. - Staphylococcus aureus is a common cause of infection with natural valves. - splinter hemorrhages are occasionally seen in people with endocarditis. - infection may occur on prosthetic valves. The left-sided heart valves are most commonly involved in this condition although tricuspid and, less often, the pulmonary valve are associated in endocarditis seen in intravenous drug users. These are the valves on the right side of the heart and receive venous blood from the main circulation. Staphylococcus aureus is a cause of endocarditis on natural heart valves, particularly in injecting drug users. Prosthetic heart valves may be involved in endocarditis, often caused by coagulase-negative staphylococci such as Staphylococcus epidermidis. Splinter hemorrhages are rarely seen now but are formed by septic emboli from vegetations lodging there. Negative blood cultures certainly do not exclude this type of infection.

Question 161

Characteristic changes in the CSF in virus meningitis include:

Answer 161

- an increase in the number of lymphocytes. The appearance of CSF in viral meningitis is slightly opaque, not turbid. In virus meningitis, glucose levels remain near normal, and there is only a slight rise in the protein level. An increase in polymorphonuclear leukocytes (neutrophils) is associated with bacterial meningitis, although an increase in the number of lymphocytes is seen with TB meningitis and that caused by the microfungus Cryptococcus neoformans as well as in virus meningitis.

Question 162

Agents that may be used to treat mycobacterial infection include:

Answer 162

- cycloserine - isoniazid - rifampicin - streptomycin Griseofulvin is an antifungal antibiotic. All the other agents have been used to treat mycobacterial infection.

Question 163

Characteristic changes in the CSF in acute bacterial meningitis include:

Answer 163

- a turbid appearance. - a marked rise in protein level. - an increase in the number of polymorphonuclear leucocytes (neutrophils). - a marked decrease in glucose level. The appearance of CSF in acute bacterial meningitis is turbid, not turbid. In acute bacterial meningitis, glucose levels drop dramatically, not only because of bacterial metabolism but also because of changes in brain chemistry following infection, and there is a significant rise in the protein level. An increase in polymorphonuclear leucocytes (neutrophils) is also associated with bacterial meningitis, although an increase in the number of lymphocytes is seen with TB meningitis and that caused by the microfungus Cryptococcus neoformans as well as in virus meningitis.

Question 164

The central nervous system may be affected by infection with:

Answer 164

- herpes simplex virus - mumps virus - human immunodeficiency virus - measles virus - poliomyelitis virus

Question 165

Causes of meningitis include:

Answer 165

- Naegleria fowleri - Mycobacterium tuberculosis - Cryptococcus neoformans - Coagulase-negative staphylococci All can cause meningitis. The coagulase-negative staphylococci cause meningitis often associated with implanted medical devices such as 'shunts'. Cryptococcus neoformans is a yeast that causes meningitis in the immunocompromised, particularly people with AIDS. Naegleria fowleri is an amoeba that causes chronic meningitis after inhalation of cysts that release amoebae that burrow into the base of the brain. One of the extra-pulmonary sites of infection with Mycobacterium tuberculosis is the meninges, leading to meningitis.

Question 166

Meticillin-resistant Staphylococcus aureus (MRSA):

Answer 166

- is often resistant to many anti-staphylococcal antibiotics. - is usually susceptible to vancomycin. - may cause asymptomatic colonization of healthy people. Isolates of MRSA resist meticillin through the production of an extra penicillin-binding protein that is not susceptible to the β-lactam antibiotics; the side chain of the meticillin molecule prevents its hydrolysis by β-lactamases. Isolates of MRSA are resistant to many other anti-staphylococcal antibiotics but are usually sensitive to vancomycin. They were originally confined to hospitalized patients but more virulent strains have evolved that are capable of causing infection in otherwise healthy individuals in the community. Many patients and staff exposed to these bacteria may, nevertheless, be colonized asymptomatically in the nose, axillae, and groins.

Question 167

Antibacterials acting on bacterial protein synthesis include:

Answer 167

- aminoglycosides - chloramphenicol Quinolones act on DNA packaging, penicillins and cephalosporins act on cell wall synthesis and bacterial protein synthesis is inhibited by aminoglycosides and chloramphenicol.

Question 168

Mechanisms that may result in microbial drug resistance include:

Answer 168

1. exclusion of the drug from its target. 2. modification of the drug. 3. alteration to the target for the drug. 4. absence of a suitable target for the drug. 5. use of alternative pathways or enzyme(s) for microbial metabolism. Each of the above mechanisms has been shown to result in microbial drug resistance. Examples of resistance mechanisms include the following. Methylation of the bacterial ribosome may result in resistance to macrolides. Chlamydias are resistant to inhibitors or peptidoglycan synthesis because they do not have cell walls. The outer membrane of most Gram-negative bacteria act as a permeability barrier to erythromycin and tetracyclines may be actively pumped out of resistance cells. Many clinically important drug resistances involve enzymatic modification of the drug. These include β-lactamases, chloramphenicol acetyltransferases and aminoglycoside acetyltransferases, phosphotransferases and adenylases. Production of alternative dihydrofolate reductase confers resistance to trimethoprim.

Question 169

The following are properties of bacterial endotoxin:

Answer 169

1. it has a pyrogenic effect. 2. it is a component of the outer membrane of the Gram-negative cell wall. 3. Lipid A is the active part of the structure. 4. it can cause circulatory collapse. Bacterial endotoxin is a lipopolysaccharide that forms part of the Gram-negative bacterial cell wall. Lipid A forms the highly conserved part of the structure and it is this that is the active component in causing disease, leading to circulatory collapse. The pyrogenic effect of endotoxin is mediated indirectly by Interleukin-1 (IL-1 ) and tumor necrosis factor-α (TNFα).

Question 170

A turbid CSF sample is examined. It has a greatly elevated protein level and the concentration of glucose is markedly reduced compared with the peripheral blood glucose level. Turbidity is due to the presence of a large number of polymorphonuclear leukocytes (neutrophils) and Gram staining revealed the presence of lanceolate Gram-positive cocci. The cause of meningitis in this case is:

Answer 170

- Streptococcus pneumoniae. The leukocyte response in meningitis caused by Mycobacterium tuberculosis results in an increase in lymphocytes. Escherichia coli, Haemophilus influenzae and Neisseria meningitidis are Gram negative. In this case, the cause is Streptococcus pneumoniae.

Question 171

The first person to use benzyl penicillin as a therapeutic agent to treat human infection was:

Answer 171

- Howard Florey Edward Jenner first vaccinated people. Joseph Lister used fungal extracts to irrigate wounds. These may have included penicillins. Ernest Duchesne probably discovered the effects of penicillin. Fleming first formally noticed the effect of the Penicillium mould on bacteria, but did not follow up his observations to apply the extract therapeutically. Florey, Chain and others first used benzyl penicillin to treat a policeman dying of overwhelming staphylococcal sepsis. Because of it scarcity during the early years of the war, decisions regarding the category of patient to be treated with the new drug were taken at the highest level, by Churchill himself.

Question 172

The β-lactamases may confer resistance to:

Answer 172

- cefotaxime - ampicillin Ampicillin and cefotaxime are β-lactam antibiotics that are susceptible to hydrolysis by β-lactamases. While meticillin is also a β-lactam antibiotic, it cannot be hydrolysed by β-lactamases because of its bulky side-chain. Betadine is an iodine-based antiseptic and zovirax is an anti-viral drug, neither of which are susceptible to inactivation by β-lactamase.

Question 173

Bacteria may resist antibiotics by:

Answer 173

- modifying the drug by addition of a phosphate group. - destroying the drug. - not having an appropriate target for the drug. - bypassing the target of the drug. - having a modified target that is no longer susceptible to the drug. All are mechanisms by which bacteria may resist antibiotics. Chlamydia lack peptidoglycan and so are not susceptible to those drugs that target this structure, such as the penicillins and cephalosporins. Resistance to many protein synthesis inhibitors may result from alterations in the ribosome structure. Resistance to the sulphonamides and to trimethoprim is frequently the result of bacteria producing an alternative enzyme to the ones that are targeted by these drugs. Phosphorylation of the drug is a common mechanism resulting in aminoglycoside resistance. Other chemical modifications such as acetylation and adenylation are also found commonly as resistance mechanisms. The β-lactamases are a large family of enzymes that destroy antibiotics by hydrolyzing the β-lactam bond that is essential for the mode of action of the β-lactam antibiotics.

Question 174

Meticillin-resistant Staphylococcus aureus:

Answer 174

- may be less pathogenic than its susceptible relatives. - is typically resistant to most classes of antibiotic. MRSA may be less virulent than MSSA, although some strains produce the Panton-Valentine Leukocidin and can cause infections in healthy individuals in the general community. Meticillin was developed to resist the activity of staphylococcal β-lactamase; resistance to this and related anti-staphylococcal penicillins (and, indeed, all β-lactam antibiotics) is through the production of an extra penicillin-binding protein, PBP2' (also referred to as PBP2a), which is not susceptible to inhibition by β-lactam antibiotics. PBP2' is encoded by a mobile element, SSCmec or staphylococcal cassette chromosome mec. This is harbored in a number of lineages, almost all of which are resistant to multiple classes of antibiotic, making treatment of MRSA infection very challenging.

Question 175

Viridans streptococci:

Answer 175

- can be distinguished from Streptococcus pneumoniae by a susceptibility test to optochin. - are a common cause of infective endocarditis. Viridans streptococci are α-hemolytic and form the majority component of the commensal microbiota of the mouth. They are common causes of infective endocarditis following dental procedures but are not involved in the causation of rheumatic fever. They are resistant to optochin whilst pneumococci are sensitive.

Question 176

The following microbes do not fit Koch's postulate requiring artificial culture of the pathogen:

Answer 176

- Mycobacterium leprae. - Treponema pallidum. Treponema pallidum and Mycobacterium leprae cannot be cultured on laboratory media, and thus cannot fulfil the requirement for artificial culture in Koch's postulates.