2 Pathogenesis of Bacterial Diseases (78) Flashcards
Case
A 57-year-old male farmer who resides in a rural area in Thailand presents with a 2-day history of a sudden onset of fever with chills, headache, lethargy, and confusion. His medical history is significant for diabetes mellitus, which was diagnosed 6 months ago; however, he is noncompliant to therapy. He has had recurrent subcutaneous abscesses for the past 3 months. The patient appears to be critically ill; he is disoriented, his temperature is 39°C, pulse is 140/minute, and blood pressure is 70/40 mmHg. The clinical diagnosis is septic shock; blood samples are sent for culture. Within 24 hours, the blood cultures grow slender, Gram-negative bacillus with bipolar staining and rounded ends, in addition to having a safety pin appearance. The bacterium is motile and produces small, smooth, non-hemolytic colonies on blood agar; the colonies become dry and wrinkled within 48 hours. The isolate grows on MacConkey’s medium and produces small, lactose-fermenting colonies. It is oxidase-positive, grows at 42°C, liquefies gelatin, and oxidizes various carbohydrates (e.g., glucose and lactose).
What is the most likely blood culture isolate?
1 Burkholderia pseudomallei
2 Yersinia pestis
3 Klebsiella pneumoniae
4 Pseudomonas aeruginosa
5 Burkholderia mallei
Burkholderia pseudomallei
Though all of the bacteria listed can produce septicemia, the characteristics of the isolate are suggestive of B. pseudomallei. The location and occupation of the patient and his clinical history are also suggestive of a B. pseudomallei infection. Burkholderia pseudomallei (Pseudomonas pseudomallei) is a soil saprophyte which causes melioidosis, a zoonotic disease endemic in many southeast Asian countries and in tropical Australia. It is a common cause of community-acquired sepsis in these areas. The bacterium is often found in tropical wet soil (e.g., in rice paddy fields). Farming and gardening necessitate prolonged or recurrent contact with soil; this can lead to infection, especially in individuals with risk factors (e.g., uncontrolled diabetes). Other risk factors include thalassemia, chronic renal failure, chronic lung disease, and excessive alcoholism. Human infection usually occurs through skin abrasions or by inhalation. Contamination of drinking water supplies has also been implicated in outbreaks of melioidosis in Australia. The disease may occur in the form of acute septicemia, subacute typhoid-like disease, or pulmonary infection (the most common form resembling tuberculosis). In the chronic form, there may be multiple suppurative foci with abscess formation in the skin, subcutaneous tissue, or internal organs. Skin and soft tissue infections may be sources of systemic infection; they can lead to an acute septicemic form of melioidosis. Acute melioidosis has a high fatality rate.
Long latency and reactivation can occur, as the bacillus can survive intracellularly in the reticuloendothelial system. Virulence factors of B. pseudomallei that allow evasion of killing mechanisms by the phagocytes have been characterized, and the genes associated with virulence have been identified. The virulence factors include various secretory antigens and cell-associated antigens (e.g., glycocalyx capsular polysaccharide, lipopolysaccharide, flagellin protein, pili, and a siderophore).
Laboratory diagnosis of melioidosis depends on the isolation of B. pseudomallei from various clinical specimens (e.g., blood, pus, and sputum) or by serology. Ashdown’s medium is one of the selective media used for the isolation of the bacterium from clinical specimens that are obtained from nonsterile body sites; it contains gentamicin as a selective agent. Serological tests for the diagnosis of melioidosis include an enzyme-linked immunosorbent assay (ELISA) for IgM and IgG antibodies and an indirect hemagglutination test. Rapid tests such as immunochromogenic card tests for IgM and IgG and DOT immunoassay have been developed, but not extensively tested. A monoclonal antibody latex agglutination test and Immunofluorescence test are reported to be useful for the detection of the B. pseudomallei antigen. A polymerase chain reaction (PCR)-based test has also been developed for the identification of B. pseudomallei.
Ceftazidime and imipenem are the drugs of choice. Combination therapy, along with cotrimoxazole, tetracycline, amoxicillin clavulanate, or chloramphenicol, has also been found to be beneficial. Prolonged treatment may be required. Melioidosis is a rare disease in the United States; however, cases of imported melioidosis and infection in exposed laboratory workers have been reported.
A safety pin appearance is a morphological feature of Yersinia pestis as well. This bacterium can be differentiated from B. pseudomallei; it is nonmotile, oxidase-negative, and produces non-lactose-fermenting, colorless colonies on MacConkey’s medium. Yersinia pestis produces plague, a zoonotic disease, which is transmitted by rat fleas. Septicemic plague is one of the clinical forms, either as a terminal event in bubonic or pneumonic plague, or as a primary form.
Klebsiella pneumoniae grows on MacConkey’s medium and produces large, lactose-fermenting mucoid colonies. The bacterium is nonmotile and oxidase-negative. It causes pneumonia, urinary infection, other pyogenic infections, and septicemia. Klebsiella pneumoniae and Yersinia pestis are both nonmotile and oxidase-negative.
Pseudomonas aeruginosa is a ubiquitous bacterium. It is one of the most common agents of nosocomial infections, including septicemia. It possesses characteristics that are similar to B. pseudomallei.; it is an oxidase-positive, actively motile, slender Gram-negative bacterium. It does not show the safety pin morphology. It produces colored colonies due to the production of pigments (e.g., pyocyanin, fluorescin, or pyorubrin). It is often hemolytic on blood agar and on MacConkey’s medium and produces non-lactose-fermenting colonies. It utilizes glucose (not lactose) oxidatively.
Burkholderia mallei (Pseudomonas mallei) resembles B. pseudomallei, but differences are in nonmotility, the inability to form acid from lactose, and the inability to liquefy gelatin. It is the causative agent of glanders, a disease primarily of equine animals. Human infection acquired from infected animals is rare and occupational; it is generally found in hostlers and veterinarians. Laboratory cultures are highly infectious.
B. pseudomallei, B. mallei, and Yersinia pestis are considered to be potential biological weapons.
Case
A 30-year-old male patient presents with a 1-hour history of severe nausea and vomiting. Before falling ill, he was at a party where he ate pudding and other food. Physical examination reveals an unremarkable temperature with mild diffuse tenderness of the abdomen. The organism isolated is a Gram-positive coccus that occurs in grape-like clusters; it is catalase- and coagulase-positive, and it forms a golden-yellow colony on agar.
What toxin released by the causative organism is responsible for the patient’s symptoms?
1 Alpha toxin
2 Exfoliatin toxin
3 Enterotoxin
4 Leukocidin
5 Toxic shock syndrome toxin (TSST-1)
Enterotoxin
Enterotoxin produced by Staphylococcus aureus is an exotoxin; it is responsible for this patient’s condition. The clinical presentation and the laboratory findings (short incubation period of 1-6 hours with predominant emesis) suggest Staphylococcal food poisoning. S. aureus organisms are Gram-positive cocci that occur in grape-like clusters. They are catalase- and coagulase-positive, and they form golden-yellow colonies on agar. Staphylococcal food poisoning results from the ingestion of preformed enterotoxins on food contaminated with S. aureus. Bacteria growing in carbohydrates and meat products produce enterotoxins that, upon ingestion, diffuse into the circulation and cause emesis by stimulating the vomiting center in the central nervous system.
Toxic shock syndrome toxin (TSST-1) has superantigen activity, and it causes life-threatening toxic shock syndrome when expressed systemically. The clinical presentation includes fever, hypotension, and diffuse macular erythema; at least three organ systems (gastrointestinal, renal, hepatic, musculoskeletal, nervous) are involved.
Exfoliatin toxin (ET) elaborated by S. aureus causes scalded skin syndrome, manifested as widespread blistering and loss of the epidermis, revealing a red base. ETA and ETB are the two antigenically distinct forms of the toxin. ET has esterase and protease activity, which targets a protein involved in maintaining epidermal integrity, causing separation of the epidermis.
Leukocidin is a membrane-damaging toxin expressed by S. aureus that acts on polymorphonuclear leukocytes.
Alpha toxin (alpha-hemolysin), the most potent membrane-damaging toxin of S. aureus, causes hemolysis.
Case
In the medical intensive care unit (MICU) of a 300-bed hospital, 5 patients developed ventilator-associated pneumonia (VAP) over a period of 1 month. All patients who developed VAP had been in the ICU for more than 7 days. In the ICU, third generation cephalosporins were commonly used for empiric Gram-negative coverage and vancomycin for Gram-positive coverage.
Purulent respiratory secretions collected from the patients were processed in the microbiology laboratory. Smear examination showed numerous polymorphs and a high predominance of intra- and extracellular Gram-negative diplococcal and coccobacillary forms. Cultures of all samples yielded pure growth of Gram-negative bacteria with identical characteristics. All the 5 isolates belonged to the same bacterial species, indicating the possibility of an outbreak. The characteristics that helped presumptive identification were microscopic appearance as Gram-negative diplococcal and coccobacillary forms (1-1.5microns x 1.5-2.5 microns in size), absence of motility, ability to grow on ordinary culture media, production of non-hemolytic large, whitish, smooth, round colonies on sheep blood agar; negative oxidase, positive catalase; absence of fermentative activity; production of acid from glucose oxidatively; and inability to reduce nitrate. All isolates had identical antibiotic susceptibility patterns. Multidrug-resistance was observed, the isolates being resistant to most of the beta-lactam antibiotics including third generation cephalosporins and most of the aminoglycosides, including gentamicin and tobramicin. Susceptibility was seen to imipenem and amikacin.
What bacteria is the most likely agent of the VAP outbreak in the MICU?
1 Serratia marcescens
2 Klebsiella pneumoniae
3 Pseudomonas aeruginosa
4 Acinetobacter baumannii
5 Moraxella catarrhalis
Acinetobacter baumannii
The characterstics of the isolates are in favor of Acinetobacter baumannii. Acinetobacter baumannii is an opportunistic pathogen. Nosocomial outbreaks including outbreaks of ventilator-associated pneumonia (VAP) caused by this bacterium are increasingly reported. Outbreaks are usually associated with multidrug-resistant strains. A. baumannii occurs as a saprophyte in soil and water and is often present in the hospital environment. It can occasionally be isolated from human skin and mucous membranes. The bacterium can survive on dry inanimate surfaces for months.
VAP is defined as nosocomial pneumonia in a patient on mechanical ventilatory support (by endotracheal tube or tracheostomy) for a period of 48 hours or more. In mechanically ventilated patients, aspiration of organisms colonizing the orophrynx is the main route of entry of bacteria to the lower respiratory tract. The lower respiratory tract can become colonized by potential pathogens from endogenous and exogenous sources. Colonization and infection by endogenous organisms like methicillin sensitive staphylococcus, S. pneumoniae, and unencapsulated Haemophilus influenzae are seen within the initial 4-5 days. Early-onset VAP is caused mainly by these bacteria. Prior antibiotic therapy and prolonged intubation favors colonization and infection by multidrug-resistant Gram-negative bacteria. A patient who has been under mechanical ventilation for more than 7 days and receiving broad spectrum antibiotics is at the risk of getting late-onset VAP caused by more resistant exogenous flora like Pseudomonas aeruginosa, Acinetobacter baumannii, and methicillin-resistant Staphylococcus aureus (MRSA). Outbreaks can originate from common environmental sources like the room humidifiers and vaporizers or any other device. Transmission may occur by the hands of health care workers if the hand washing policy is not strictly followed. During an outbreak investigation, different possible sources are screened for the bacterium and all isolated strains are identified by molecular typing.
Antibiotic resistance in acinetobacter is acquired by exposure to antibiotics rather than being inherent. In the hospital environment, chromosomal cephalosporinase production facilitated by selective pressure for beta-lactam use confers cephalosporin resistance to Acinetobacters. Carbapenems (imipenem and meropenem) are widely used for the treatment of infection by such strains. Carbapenem resistance may develop as a result of altered outer membrane proteins impairing permeability of cell membrane or alterations in the penicillin-binding proteins. Recently, outbreaks caused by carbapenem-resistant clones of Acinetobacter baumannii have been reported from various countries including the U.S. Carbapenem resistance due to production of specific carbapenem-hydrolyzing beta lactamases (carbapenemases) has been detected as a characteristic of these clones. Colistin is found to be of use in treating infections by the carbapenem-resistant strains. Ampicillin-sulbactam is considered as a cost-effective therapeutic option for treatment of severe infections due to multidrug-resistant A. baumannii. In vitro susceptibility of multidrug-resistant A.baumannii to a new glycylcycline antibiotic, Tigecycline, has been recently reported.
Serratia marcescens is a Gram-negative bacillus widely distributed in nature and belongs to enterobacteriaceae. It has emerged as an important nosocomial pathogen in the last few years. It can be differentiated from Acinetobacter by its ability to reduce nitrate to nitrite and fermentation of carbohydrates. Some strains produce a pigment prodigiosin that gives a red color to the colonies. Presence of pigment-producing strains in sputum can give a red color to the sputum resembling hemoptysis. Nosocomial colonization and outbreaks of infections are mostly caused by multidrug-resistant strains.
Klebsiella pneumoniae is a short Gram-negative, capsulated, nonmotile rod capable of causing a wide variety of community as well as hospital-acquired infections. Multidrug-resistant strains occur in the hospital environment and cause nosocomial infections including VAP. It belongs to enterobacteriaceae and, unlike A. baumannii, reduces nitrate to nitrite and ferments various carbohydrates with production of acid and gas.
Pseudomonas aeruginosa, a ubiquitous Gram-negative bacillus, is one of the most common nosocomial pathogens with intrinsic as well as acquired resistance to various antibiotics. The bacterium can be differentiated from Acinetobacter by its active motility, positive oxidase test, and production of pigments.
Moraxella catarrhalis is a member of the normal flora of the upper respiratory tract. It is usually associated with opportunistic community-acquired infections like bronchitis, pneumonia, sinusitis, and otitis media. Morphologically it resembles Acinetobacter baumannii, is a non-fermenter, and it can be differentiated by positive oxidase test.
A 2-month-old baby was brought to the pediatric clinic situated in one of the developing countries in South America. He was diagnosed with congenital heart disease. The infant’s mother had history of mild febrile illness with rashes during the first trimester of her pregnancy, suggestive of rubella infection. Which of the following tests is most likely to be used for routine diagnosis of congenital rubella infection in the baby?
1 Isolation of rubella virus from urine using vero cells
2 Isolation of rubella virus from throat secretions using rabbit kidney cells
3 Detection of rubella RNA in serum by PCR-based test
4 Demonstration of rubella-specific IgG in serum
5 Demonstration of rubella-specific IgM in serum
Demonstration of rubella-specific IgM in serum
Congenital rubella infection is routinely diagnosed by serological demonstration of rubella-specific IgM antibodies. The presence of IgM antibodies indicates immune response in the fetus, whereas presence of IgG antibodies in a 2-month-old infant may be due to passive transfer of maternal antibodies. Virus isolation is not routinely used for diagnosis because of the difficulties and delay involved.
PCR-based test for detection of rubella virus RNA is not available for routine diagnostic use.
Rubella or German measles is a mild exanthematous fever characterized by transient macular rash and lymphadenopathy. The infection is harmful only when it occurs in a pregnant woman especially in the first trimester of pregnancy. The extent of teratogenic effects produced by the virus depends on the timing of infection. The earlier the maternal infection, the more fetal damage, resulting in death or congenital malformations of the fetus. The classical triad of congenital rubella syndrome consists of cataract, deafness, and cardiac defects. Intellectual disability, hepatosplenomegaly, thrombocytopenic purpura, and central nervous system involvement are other manifestations constituting expanded rubella syndrome.
Rubella virus belongs to Togaviridae family and is the only member of genus Rubivirus. The viral particle is 50-70 nm in diameter with single stranded RNA genome surrounded by an envelope carrying hemagglutinin peplomers.
Laboratory diagnostic methods for rubella include virus isolation and serology. Serological diagnosis is routinely used. Enzyme Liked Immunosorbent Assay (ELISA) is used for detection of IgM and IgG antibodies. In a pregnant woman with suspected rubella, IgM antibody alone without IgG means current acute infection. IgG antibody alone without IgM is suggestive of past infection or vaccination and indicates immunity.
In congenital rubella, viral excretion may last for several months after birth. The virus may be isolated from a variety of sources such as urine, throat secretions, bone marrow, or cerebrospinal fluid. Rabbit kidney or Vero cells are used for virus isolation.
The virus grows better if cultures are incubated at lower temperature, around 33-35°C. Cytopathogenic effect produced by rubella virus is inconspicuous. Immunofluorescence may be used for detection of viral antigens in cell cultures 3-4 days after inoculation.
Detection of IgG antibodies can be used for diagnosing congenital rubella infection in infants more than 6 months old.
Reverse Transcriptase Polymerase chain reaction (RT-PCR) assay has been used for detection of rubella RNA in serum samples during epidemiological surveillance for congenital rubella syndrome. RT-PCR assay has also been used to detect rubella genome in oral fluids, throat swabs and tissue samples. Acute cases within 2 days after onset of symptoms could be diagnosed by this assay, even before IgM response reached detectable levels. The test has been successfully used for diagnosis of prenatal congenital rubella infection by detection of rubella genome in amniotic fluid.
Since 1969, the use of attenuated live rubella vaccine has considerably reduced the incidence of congenital rubella in developed countries and some developing countries.
Case
A 27-year-old male accident victim with a head injury is admitted to the ICU and kept on mechanical ventilatory support. On the seventh day after admission, he is clinically diagnosed with pneumonia. Blood samples and lower respiratory secretions are submitted to the laboratory for culture; empiric antimicrobial therapy is started.
What is the most likely etiologic agent of pneumonia in this patient?
1 Streptococcus pneumoniae
2 Klebsiella pneumoniae
3 Mycoplasma pneumoniae
4 Moraxella catarrhalis
5 Haemophilus influenzae
Klebsiella pneumoniae
The correct response is Klebsiella pneumoniae.
The patient had been on mechanical ventilation for >5 days when he developed pneumonia; by definition, he has nosocomial late-onset ventilator-associated pneumonia (VAP). The main etiological agents of late-onset VAP consist of antibiotic-resistant gram-negative bacilli and methicillin-resistant Staphylococcus aureus. The most common gram-negative bacilli associated with late-onset VAP are Pseudomonas, Klebsiella, Enterobacter, Serratia, and Acinetobacter species. Often, more than one organism is involved.
From the given list of bacteria, the most likely etiological agent of pneumonia in this patient is Klebsiella pneumoniae. Multi-drug resistant strains of Klebsiella pneumoniae occur in the hospital environment and are known to be important agents of nosocomial infections, including late-onset VAP.
Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis are generally associated with community-acquired pneumonia, but colonization and infection by these endogenous bacteria can occur within the initial 4-5 days of ventilatory support (resulting in early-onset VAP).
Mycoplasma pneumoniae is an important agent of community-acquired pneumonia. Recently, the possible involvement of this organism in early-onset VAP has been suggested.
Staphylococcus epidermidis is normally found on the skin but may also be found in surface wounds and abrasions without being the true etiologic agent of the infection. In this case, the organism is said to:
1 Infect the wound.
2 Parasitize the wound.
3 Colonize the wound.
4 Infiltrate the wound.
5 Contaminate the wound.
Colonize the wound.
The correct answer is colonize the wound. Both coagulase-positive and coagulase-negative staphylococci are normal flora on the skin of healthy individuals, and their presence in a surface lesion must be carefully considered in light of other clinical factors. Generally, the coagulase-negative staphylococci, including S. epidermidis, are considered only harmless colonizers, especially in the presence of other normal skin flora. There are cases, however, where the coagulase-negative staphylococci are considered etiologic agents, especially in surgical wound infections where they may be the only agent cultured.
The term “contamination of a wound” may or may not imply true infection and is not often used to describe the role of a particular organism in a lesion. Infection implies multiplication and spreading of the agent, usually with accompanying exudate of polymorphonuclear leukocytes. Infiltration and parasitization are not usually used to describe the relationship of an organism to a particular wound site, although organisms may in fact infiltrate into tissue thereby causing infection.
Diphtheria toxin, produced by Corynebacterium diptheria acts by
1 Inhibiting protein synthesis by ADP-ribosylation of elongation factor 2 (EF-2)
2 Inducing TNF
3 Inducing interleukin-1 production
4 Degrading cell walls
Inhibiting protein synthesis by ADP-ribosylation of elongation factor 2 (EF-2)
Diphtheria toxin, produced by Corynebacterium diptheria , inhibits protein synthesis by ADP-ribosylation of elongation factor 2 (EF-2). This activity depends on two functions mediated by different domains of the molecule. Fragment A, a 22,000 molecular weight peptide at the amino-terminal end is the enzyme that catalyzes the transfer of ADP-ribose from nicotinamide adenine dinucleotide (NAD) to EF-2, thereby inactivating it. The ADP-ribosylation freezes the translocation complex, and protein synthesis stops.
Tetanus toxin acts by
1 Its neurotoxic effect in preventing the release of the inhibitory neurotransmitter glycine
2 Blocking the release of acetylcholine
3 Stimulating adenylate cyclase activity
4 Increasing cAMP production
Its neurotoxic effect in preventing the release of the inhibitory neurotransmitter glycine
Tetanus toxin, produced by Clostridium tetani , is a neurotoxin that prevents release of the inhibitory neurotransmitter glycine. This causes muscle spasms. The toxin (tetanospasmin) is composed of two polypeptide subunits encoded by plasmid DNA. The heavy chain of the polypeptide binds to gangliosides in the membrane of the neuron; the light chain exerts the toxic activity.
Verotoxin
1 Is an endotoxin produced by E.coli (O157:H7 serotype)
2 Is an exotoxin produced by E.coli (O157:H7 serotype)
3 Is produced by Streptococcus viridans
4 Is produced by Clostridium tetani
Is an exotoxin produced by E.coli (O157:H7 serotype)
Verotoxin is an exotoxin produced by strains of E.coli with the O157:H7 serotype. These strains cause bloody diarrhea and are the cause of outbreaks associated with eating undercooked hamburgers. The toxin is named for its toxic effect on Vero (monkey) cells in culture.
The heat-labile enterotoxin produced by E.coli causes diarrhea by
1 Stimulating adenylate cyclase activity in cells in the small intestine
2 Stimulating guanylate cyclase
3 Blocking protein synthesis
4 Blocking release of acetylcholine
Stimulating adenylate cyclase activity in cells in the small intestine
The heat-labile enterotoxin produced by E.coli causes diarrhea by stimulating adenylate cyclase activity in cells in the small intestine. The resulting increase in the concentration of cyclic adenosine monophosphate (cAMP) causes excretion of the chloride ion, inhibition of sodium ion absorption, and significant fluid and electrolyte loss into the lumen of the gut.
The botulinum toxin acts by:
1 Blocking the release of acetylcholine at the synapse and thereby producing paralysis
2 Affecting cGMP activity
3 Blocking protein synthesis
4 Activating the complement cascade
Blocking the release of acetylcholine at the synapse and thereby producing paralysis
Botulinum toxin produced by Clostridium botulinum is a neurotoxin that blocks the release of acetylcholine at the synapset, producing paralysis. The genes for this toxin are encoded by a temperate bacteriophage. It is composed of 2 polypeptide subunits held together by disulfide bonds and one of the subunits binds to a receptor on the neuron.
Toxic shock syndrome toxin (TSST) is produced by
1 Bordetella pertussis
2 Staphylococcus aureus
3 Bacillus anthracis
4 Escherichia coli
Staphylococcus aureus
Toxic shock syndrome toxin (TSST) is produced by certain strains of Staphylococcus aureus. TSST binds directly to class II major histocompatibility (MHC) proteins without intracellular processing. This complex interacts with the β-chain of the T cell receptor of many helper T cells and causes the release of large amounts of interleukins, which then produce many of the signs of the toxic shock.
The ability of an organism to produce pathologic changes or disease in the host is known as
1 Virulence
2 Inflammation
3 Toxigenicity
4 Pathogenicity
5 Invasiveness
Pathogenicity
The capacity of pathogens to cause disease depends on their abilities to infect the host or protect themselves against the body’s defenses, to invade and multiply in tissues and to cause damage to the tissue.
Virulence is the measure of degree of pathogenicity and can be defined as the sum of infectivity, invasiveness and toxigenicity.
Inflammation is a nonspecific pathologic process consisting of a dynamic complex of cytologic and histologic reactions that occur in response to an injury or abnormal stimulation by a physical, chemical, or biologic agent.
Toxigenicity is the ability of the organism to produce toxins.
Invasiveness is the ability of the organism to penetrate and grow in the host tissue.
Substances that stick to a microorganism surface and make it more attractive to phagocytic cells are known as:
1 Interferons
2 Bacteriocins
3 Lysozymes
4 Opsonins
5 Phagosomes
Opsonins
Opsonins stick to the surface of a microorganism and enhance phagocytosis.
Interferons are the class of small, antiviral glycoproteins produced by cells infected with an animal virus.
Bacteriocins are proteins produced by certain bacteria that can kill other bacteria.
Lysozyme is a digestive enzyme found in lysozymes, tears, and other body fluids and is destructive to bacterial cell walls.
Phagosome is a membrane bound vesicle containing an ingested particle and is found in phagocytic cells.
Select the most true statement relating to exotoxins
1 Produced by gram negative bacteria only
2 Lipopolysaccharide in composition
3 Antibodies do not neutralize toxicity
4 Never contain ketodeoxyoctonate (KDO)
5 Toxicity resides in lipid A component
Never contain ketodeoxyoctonate (KDO)
Exotoxins are protein produced by gram negative and gram positive bacteria. Antibodies neutralize their toxicity and never contain ketodeoxyoctonate (KDO). Toxicity is due to biologic action of protein. Toxicity resides in the lipid A component in endotoxins.
Endotoxins are
1 Mostly heat labile
2 Always highly toxic in microgram quantities
3 Known as lipopolysaccharides
4 Used to form toxoids
5 Produced by gram positive bacteria only
Known as lipopolysaccharides
Endotoxins are lipopolysaccharide portion of the cell wall produced by gram negative bacteria only. They are heat stable and cannot be used to form toxoids. Endotoxins are toxic in milligram quantities, while most exotoxins are toxic in microgram quantities.
Endotoxin stimulates the fever center in the hypothalamus and causes hypotension. It initiates coagulation that can result in intravascular coagulation.
The enzyme that enables an organism to clot plasma and form a fibrin coat is
1 Streptokinase
2 Hyaluronidase
3 Staphylokinase
4 Coagulase
5 Collagenase
Coagulase
Coagulase is the exoenzyme produced by Staphylococcus aureus, which causes citrated blood plasma to clot in the laboratory. However, kinases dissolve fibrin clots, thus enabling the organism to invade and spread throughout the body.
Hyaluronidase enables pathogen to spread through connective tissue by breaking down hyaluronic acid, the “cement” that holds tissue cells together.
Collagenase breaks down collagen, the supportive protein found in tendons.
The ability of pathogens to invade, infect, cause damage and produce disease in the host is termed as
1 Pathogenicity
2 Toxigenicity
3 Virulence
4 Infection
5 Attenuation
Virulence
Virulence is the ability of a pathogen to invade, infect, cause damage and produce disease in the host.
Pathogenicity is the ability of an organism to produce pathologic changes or disease in the host.
Toxigenicity is the ability of the organism to produce toxins.
Infection occurs when a pathogenic microbe is able to multiply in the tissues where it lodges.
Attenuation is the process by which the microorganism is rendered less pathogenic or avirulent.
Diphtheria toxin, that is produced by the bacterium Corynebacterium diphtheriae, exerts its action by which of the following?
1 Blocking release of acetylcholine
2 Blocking release of inhibitory neurotransmitters
3 Inhibiting protein synthesis by ADP ribosylation of elongation factor 2
4 Acting as an adenylate cyclase
5 An unknown mechanism
Inhibiting protein synthesis by ADP ribosylation of elongation factor 2
Diphtheria toxin binds to the receptors on the cell membranes via a region near its carboxyl end. This is followed by transport across the cell membrane, a proteolytic nick and reduction of the disulfide bond. This in turn releases active fragment A, which inactivates EF-2. The following chemical reaction occurs:
EF-2 + NAD —–> EF-2-ADP-ribose + Nicotinamide + H+
Clostridium botulinum exotoxin blocks release of acetylcholine. Clostridium tetani exotoxin interferes with the release of inhibitory neurotransmitters such as glycine and gamma-aminobutyric acid. One of the toxins of Bacillus anthracis is an adenylate cyclase
One of the biological effects of endotoxins is fever. This is caused due to release of
1 Bradykinin
2 Interleukin-1
3 Alternative pathway of complement C3a and C5a
4 Hageman factor
5 Virus
Interleukin-1
Interleukin-1 is released by macrophages; it acts on the hypothalamic temperature regulatory center and causes fever. Bradykinin causes hypotension (shock). The alternative pathway of complement C3a and C5a causes inflammation. Hageman factor initiates coagulation. Endotoxins do not release viruses.
Aflatoxin is a toxin produced by an organism that is not generally invasive in humans although the toxin itself may be extremely dangerous. What is the organism most likely to produce this toxin?
1 Acinetobacter baumanii
2 Agrobacterium radiobacter
3 Aspergillus flavus
4 Actinomyces naeslundii
5 Acremonium sp
Aspergillus flavus
Aflatoxin is a fungal toxin that is produced by Aspergillus flavus and Claviseps purpurae. The other organisms listed, with the exception of Acremonium sp., are bacteria and are not producers of exotoxins. Normally these aflatoxins make their way into humans through their contamination of various foods. Aflatoxin is actually named from its major source, Aspergillus flavus.
Ergot poisoning is an infrequent but significant contributor to illness due to the toxin ergotamine. The most likely source of this toxin would be
1 Bacillus cereus toxin contaminating cooked rice
2 Staphylococcus aureus toxin contaminating cream-based foods
3 Histoplasma capsulatum toxin contaminating herbs
4 Entamoeba histolytica toxin contaminating water
5 Claviceps purpurae toxin contaminating wheat and rye products
Claviceps purpurae toxin contaminating wheat and rye products
Ergotamine is a fungal toxin produced by Claviceps sp. Humans are poisoned by eating contaminated grain such as wheat and rye.
Of the organisms listed, which one produces a neurotoxin?
1 Bacillus cereus
2 Clostridium perfringens
3 Escherichia coli
4 Clostridium botulinum
5 Vibrio cholerae
Clostridium botulinum
C. botulinum produces a potent neurotoxin that causes botulism, a potentially life-threatening food-borne illness exhibiting neurologic symptoms. The other organisms listed produce an enterotoxin that results in intestinal symptoms also resulting from ingestion.
The exotoxin responsible for this disease is produced by a noninvasive organism. The toxin activates the catalytic subunit of adenylate cyclase, resulting in persistent production of cAMP because the control reaction (GTP hydrolysis by GTPase) is no longer operative. The continuous synthesis of cAMP provokes the movement of water and electrolytes across the intact epithelial cells into the lumen of the small intestine producing diarrhea. The most likely diagnosis resulting from this mechanism of action is:
1 Food poisoning
2 Cholera
3 Bacillary dysentery
4 Amebic dysentery
5 Pseudomembranous colitis
Cholera
The toxin described is unique to Vibrio cholerae . Other enterotoxins act with a different mechanism
Endotoxin is the
1 Protein component of the cell well of gram negative bacteria
2 Protein component of the cell wall of gram positive bacteria
3 Lipopolysaccharide component of the outer membrane of gram positive bacteria
4 Lipopolysaccharide component of the outer membrane of gram negative bacteria
5 Lipoprotein component of the membrane of both gram positive and gram negative bacteria
Lipopolysaccharide component of the outer membrane of gram negative bacteria
Only gram negative bacteria have an outer membrane, and it is this component that is referred to as endotoxin. The toxicity of endotoxin varies greatly among these organisms and resides in the Lipid A portion.
The limulus lysate test is used in both industry and medicine as a sensitive procedure to detect the presence of even picogram amounts of
1 Viral DNA
2 Bacterial endotoxin
3 Bacterial exotoxin
4 Parasitic exoantigens
5 Fungal exotoxins
Bacterial endotoxin
A commonly used screening test for endotoxin is the limulus lysate test. This test is based on the observation that amebocytes from the horseshoe crab, limulus polyphemus, clot in the presence of even picogram amounts of bacterial endotoxin. The limulus test has formerly been used in the clinical laboratory to screen spinal fluid for endotoxin as an indication of the presence of gram negative bacteria. It is not a routine procedure but may be available at reference laboratories.
Which of the following organisms would be considered invasive
1 E. coli O157:H7
2 Vibrio cholerae
3 Corynebacterium diphtheriae
4 Salmonella typhi
5 Shigella dysenteriae
Salmonella typhi
All the organisms listed, with the exception of S. typhi, produce disease via a toxin. S. typhi invade through the intestinal mucosa, beyond the lamina propria, and ultimately into the blood stream and back into the gall bladder where this organism may remain for the life of the patient unless the gall bladder is removed.
To quickly determine if an invasive diarrheal pathogen or a toxin producer is present in a stool specimen, one may
1 Stain the specimen and look for white blood cells, which would indicate an invasive process
2 Culture the specimen and observe for beta-hemolytic isolates, which would indicate toxin producers
3 Perform a limulus lysate test on the specimen, which would detect endotoxin
4 Treat the specimen with alcohol to kill all but spore-forming bacteria, then culture for the presence of these agents
5 Stain a pure culture isolate with the flagella stain, which would indicate invasiveness in the presence of these flagella
Stain the specimen and look for white blood cells, which would indicate an invasive process
The presence of white blood cells in a stool specimen often indicates an invasive process in cases of diarrhea; whereas, toxin-mediated diarrhea is usually not accompanied by white blood cells in the feces. Beta-hemolysis is not an indicator of toxicity, nor do flagella make an organism invasive. Because the feces is heavily colonized by gram negative bacteria, one would expect the limulus lysate test (if it were performed) to be positive; however, this test is never performed on stool specimens because all tests would be positive. Alcohol treatment of stool can be used to assist in the culture of Clostridioides difficile by killing most vegetative cells and leaving bacterial spores unharmed. However, this procedure still does not indicate a mechanism of pathogenicity.
In the laboratory, a sputum culture from an adult patient experiencing respiratory symptoms, revealed the presence of Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus, all in relatively low numbers. Because the patient may have pneumonia one of the organisms cultured, may likely be the etiologic agent. Other than initiating empiric therapy, what would you do next to help confirm your suspected diagnosis?
1 Ask the lab to confirm the identification of the organisms by either enzyme immunoassay tests or conventional biochemical tests
2 Order acute and convalescent sera, drawn for subsequent serologic testing, looking for a 4-fold rise in titer to one of the agents
3 Review the gram stain results to determine if the specimen was adequate, determine which morphotype predominated on the smear, then order a blood culture
4 Order a bronchoalveolar lavage specimen to confirm the results of sputum culture
5 Ask the laboratory to do a susceptibility test on all three isolates
Review the gram stain results to determine if the specimen was adequate, determine which morphotype predominated on the smear, then order a blood culture
Sputum culture remains a common request for patients with pneumonia, even though it is fraught with error. The specimen of choice for the diagnosis of pneumonia may be a blood culture, especially if the results of blood culture can correlate to the findings from sputum. The one tool available to the laboratory to validate the adequacy of the sputum specimen is the gram stain. The presence of squamous epithelial cells always indicates contamination with oropharyngeal flora. Compounding the problem of interpretation regarding the results of sputum culture, is the fact that all three of the organisms listed, S. pneumoniae, H. influenzae, and S. aureus are normal flora in the human respiratory, and they can also be pathogenic in the respiratory tract. Validation by gram stain and blood culture facilitates the interpretation of results.
Confirmation of identification or susceptibility test results would not help correlate culture results to patient outcome because the non-pathogen could be the one confirmed. Acute and convalescent sera are not used for testing these bacteria. A BAL specimen could be ordered if the sputum specimen had been inadequate or inappropriately collected, but such a procedure would add another 24-48 hours to the time for results.
For collecting a rectal specimen for the culture of Neisseria gonorrhoeae, the appropriate specimen will most likely be
1 A moistened calcium-alginate swab rubbed firmly over the entire perirectal area
2 A cotton-tipped swab inserted 1 inch into the anal canal and moved from side to side for about 15 seconds
3 A swab inserted into the rectum just beyond the anal sphincter in order to insure that an adequate amount of feces appears on the swab upon removal
4 A cotton-tipped swab used to firmly sample the external anal opening
5 A rectal swab is rarely recommended for the isolation of N. gonorrhoeae
A cotton-tipped swab inserted 1 inch into the anal canal and moved from side to side for about 15 seconds
N. gonorrhoeae is often isolated from rectal sources of homosexual men and from symptomatic women. The anal crypts must be firmly sampled by inserting the swab about an inch into the anal canal, and moving it from side to side. An external sample of the rectal or perirectal area is inadequate. Care should be taken to prevent too much feces from contaminating the swab tip during sampling.
The direct gram stain can be used effectively for the diagnosis of gonococcal urethritis in
1 Symptomatic men
2 Asymptomatic men
3 Both symptomatic and asymptomatic men
4 Asymptomatic women
5 Either symptomatic or asymptomatic women
Symptomatic men
When used properly by trained personnel, the direct gram stain is about 95% sensitive and 100% specific, for the diagnosis of gonococcal urethritis in symptomatic men. Culture of the urethra of these patients may be necessary only if the direct smear is equivocal or negative. The direct smear is not sensitive enough to detect gonococcal infection in asymptomatic men. Gram stained smears of exudate from women may be misleading since there are relatively large numbers of non-gonococcal Gram-negative rods residing as normal flora of the vagina.
Case
A 20-year-old woman presents with a malodorous vaginal discharge. She has had this discharge for approximately 2 weeks. She has been sexually inactive for the past year. She does not note any itching or abdominal/pelvic pain. Physical examination reveals a homogeneous gray discharge. There appears to be no redness or ulceration of the vulva and surrounding area. Some of the vaginal discharge is obtained and mixed with 10% KOH, producing a fishy amine odor. A Gram stain of the vaginal discharge reveals diagnostic “clue” cells. Refer to the image.
Based on the clinical presentation and test results, what is the most likely diagnosis?
1 Gonorrhea cervicitis
2 Bacterial vaginosis
3 Candidiasis/vulvovaginitis
4 Chlamydial endocervicitis
5 Trichomoniasis
Bacterial vaginosis
Bacterial vaginosis (nonspecific vaginitis) is a disease that is characterized by a malodorous vaginal discharge. The diagnosis of bacterial vaginosis is based on the presence of at least 3 of the 4 following signs:
characteristic homogeneous gray discharge
presence of “clue” cells
vaginal pH greater than 4.5
the release of a fishy amine odor from vaginal secretions mixed with 10% potassium hydroxide (KOH)
Gonorrhea cervicitis is caused by Neisseria gonorrhoeae and is a common sexually transmitted disease. Clinical manifestations include vaginal discharge, dysuria, intermenstrual bleeding, purulent or mucopurulent endocervical discharge, and erythema. Many women have mild or asymptomatic infections. The organism is a gram-negative diplococcus that is oxidase positive, glucose positive, sucrose negative, ONPG negative, and nitrate negative. When a gram-negative diplococcus is detected in the Gram-stained smear of discharge material, it is not diagnostic for a woman of Neisseria gonorrhoeae because there are other normal gram-negative diplococci present in the vaginal tract that are part of the normal flora. DNA probes, culture with biochemical identification, and EIA methods are used to detect and identify the organism.
Candidiasis/vulvovaginitis is often a chronic disease and is the most common candida infection. Candida albicans is the most common Candida species causing candidiasis. The disease causes an intense vaginal itching and burning sensation that is accompanied by a pruritus. There is usually a discharge that is described as thick and curd-like. The vulva is inflamed, and ulcerations can occur. Gram stains of the discharge will typically contain large amounts of yeast cells.
Chlamydial endocervicitis is a mucopurulent infection caused by Chlamydia trachomatis, a sexually transmitted disease. The organism cannot be detected by normal Gram staining methods and requires fluorescent antibody staining techniques, cell culture, DNA probes, or EIA methods to detect it. It is the most common sexually transmitted pathogen. The organism also causes urethritis, endometritis, salpingitis, tubal factor infertility, and ectopic pregnancy.
Trichomoniasis is caused by Trichomonas vaginalis, a flagellated protozoan. It is characterized by vaginal itching and discomfort with a discharge that is often copious, frothy, and malodorous. Person-to-person spread is primarily through sexual contact. There is severe itching of the vulva and inner thighs. Definitive diagnosis is made by demonstrating the motile organisms in fresh secretions; it can also be demonstrated in spun down urine during a urinalysis.
