6 Sporulating and Nonsporulating Gram Positive Baccili (45) Flashcards
Case
A 50-year-old industrial worker engaged in handling animal products develops a painless papule on his right forearm where he had a minor abrasion 5 days earlier. The papule became a vesicle after 48 hours and ruptured, leaving an ulcer with a black necrotic area in the center and surrounding edema. Gram stain of smear from the skin lesion showed Gram-positive bacilli (1-1.5 x 3-5 microns) that appeared encapsulated. Aerobic culture done from the lesion on 5% sheep blood agar yielded pure growth of non-hemolytic colonies 2-5 mm diameter with wavy border and ground glass appearance after overnight incubation. The isolate was non-motile, produced spores, and was sensitive to penicillin.
What is the most likely Gram-positive bacillus isolated from this patient?
1 Corynebacterium ulcerans
2 Bacillus cereus
3 Clostridium perfringens
4 Bacillus anthracis
5 Erysipelothrix rhusiopathiae
Bacillus anthracis
is the bacterium isolated. The skin lesion is likely to be acquired as a result of infection from animal products. Presumptive identification as Bacillus anthracis is supported by the demonstration of the Gram-positive large encapsulated bacilli in a smear from the lesion, its rapid aerobic growth, the characteristic appearance of the colonies, the absence of motility, and its sensitivity to penicillin. The organism is an aerobic spore-bearer that produces a poly-gamma-d-glutamic acid capsule in vivo. The capsule is anti-phagocytic and protects the bacteria from complement-mediated lysis. Spores are highly resistant and are not produced in living tissue. Carbon dioxide levels within the body inhibit sporulation. Exposure to free oxygen is necessary for formation of spores. Absence of motility and hemolysis in the culture help to differentiate the organism from other bacillus species often encountered as contaminants in culture.
Bacillus anthracis causes 3 clinical forms of anthrax: cutaneous, inhalational, and gastrointestinal.
Cutaneous form is the most common. The development and appearance of the skin lesion in the patient is characteristic of cutaneous anthrax (malignant pustule).
Inhalational anthrax is the most severe form.
Complications like septicemia and meningitis can occur. Tests used to confirm identification of the organism in reference laboratories are lysis by gamma phage, capsule-specific staining, and polysaccharide cell wall antigen staining. Polymerase Chain Reaction (PCR) for Bacillus anthracis chromosomal markers and Enzyme-Linked Immunosorbent Assay (ELISA) for antigen detection are rapid tests for the direct detection in clinical samples. Penicillin, ciprofloxacin, or doxycycline are usually recommended for treatment of anthrax. Virulence factors of bacillus anthracis are anthrax toxin and capsule. These are encoded on 2 large plasmids pXO1 and pXO2, respectively. Strains lacking either of these plasmids have greatly reduced virulence. Anthrax toxin consists of 3 protein components termed protective antigen (PA), edema factor (EF), and lethal factor (LF). Separately, none of these proteins is toxic. They act synergistically in binary combinations. PA is the cell-binding moiety, and it mediates entry of EF and LF into the host cell. EF in combination with PA forms the edema toxin. LF with PA forms the lethal toxin, which is a major virulence factor of anthrax bacillus. Anthrax vaccine for humans contains protective antigen. B. anthracisis is considered an important agent of bioterrorism because of its virulence factors and physical properties.
Corynebacterium ulcerans is a Gram-positive non-motile bacillus; it is club-shaped, non-sporing, and non-capsulated. It possesses metachromatic granules, which give the rod a beaded appearance. It produces slightly hemolytic colonies on blood agar. The organism is closely related to Corynebacterium diphtheriae and can carry the same bacteriophage that codes for diphtheria toxin; therefore, it can cause disease similar to clinical diphtheria. The organism is primarily a veterinary pathogen, and it causes mastitis in cattle and infections in other domestic animals. Sporadic cases of diphtheria caused by C. ulcerans have been reported in humans who come in contact with farm animals or consume unpasteurized dairy products. Human to human transmission is not documented. Cutaneous infection is extremely rare.
Bacillus cereus is an aerobic spore-forming bacillus related to B. anthracis, and it is a ubiquitous organism. It is motile and produces hemolysis on sheep blood agar. It produces beta-lactamase; therefore, it is resistant to penicillin and other beta-lactam antibiotics. It causes food poisoning, both the emetic type following consumption of preformed toxin in rice dishes, and the diarrheal type following consumption of contaminated meat or dairy products. B. cereus can also cause severe diseases when combined with predisposing factors in the patient, such as drug addiction, immunosuppression, and prosthetic implants. Endophthalmitis, osteomyelitis, pneumonia, and endocarditis are examples of severe infections.
Clostridium perfringens (clostridium welchii) is a large Gram-positive, nonmotile, anaerobic spore-bearing bacillus. It causes invasive infections, myonecrosis, and gas gangrene, often following wound contamination. Several toxins with lethal, necrotizing, and hemolytic properties contribute to pathogenesis. Some strains produce a powerful enterotoxin and cause food poisoning.
Erysipelothrix rhusiopathiae are Gram-positive, non-motile, non-sporing, non-capsulated slender rods arranged singly, in short chains, or forming long filaments. The organism produces transparent glistening colonies on blood agar. E. rhusiopathiae is an occupational pathogen that typically causes non-pyogenic cutaneous lesions known as erysipeloid. The organism can be cultured from a biopsy of the inflamed edge of the lesion. It is highly susceptible to penicillin and intrinsically resistant to vancomycin. Erysipeloid lesions follow inoculation of the organism at the site of abrasion, and they usually occur on the hand or fingers of persons handling animals, fish, or animal products. The lesions are painful, edematous, and erythematous; they often involve the local lymph glands. E. rhusiopathiae can cause a wide spectrum of diseases, namely polyarthralgia, septic arthritis, peritonitis, bacteremia, and endocarditis.
Case
A 29-year-old man presents 1 hour after stepping on a sharp nail. The nail penetrated deep into his foot; his last tetanus immunization was at age 6.
What treatment would be most appropriate?
1 Tetanus immunoglobulin only
2 Tetanus toxoid only
3 Tetanus immunoglobulin plus toxoid
4 Tdap vaccine plus antibiotics
5 Antibiotics only
Tdap vaccine plus antibiotics
This patient has not received a tetanus booster for over 10 years. Current immunization schedule includes a primary course of 3 doses of DTaP (diphtheria, tetanus inactivated toxoids, and acellular pertussis) given from 2 months to 5 years. A booster should be given 10 and 20 years after the primary course. If more than 5 years have elapsed since the last dose, a booster dose of Tdap (tetanus, diphtheria toxoid, acellular pertussis) is indicated. This vaccine is recommended for children over 7 and adults. Tdap is preferred to Td (tetanus and diphtheria toxoid only) because of the resurgence of pertussis infections due to waning immunity. In children under 7, DTaP or DT (if pertussis vaccine is contraindication or allergy is present) is the appropriate therapy.
In the case of a dirty wound, antibiotics are usually administered in addition to vaccination to prevent the risk of infection.
Tetanus immunoglobulin is given if the individual has not received at least 2 previous tetanus toxoid shots, or if the tetanus-prone wound has not been treated for more than 24 hours. Some patients have contraindications to Td, such as a history of allergy or adverse vaccine reaction, unstable neurological condition, or acute illness. In these patients, passive immunization with tetanus immunoglobulin may be indicated.
Tetanus toxoid only is adequate to prevent the development of tetanus from the dirty wound but does not address the fact that the patient also requires a diphtheria and acellular pertussis booster. In the context of an acute wound, antibiotics are usually administered as a precaution along with tetanus toxoid.
Only prescribing antibiotics is not appropriate, as it fails to address the patient’s need for boosters to prevent tetanus and diphtheria.
Case
A 45-year-old woman is undergoing chemotherapy for malignant melanoma, which was diagnosed 6 months earlier. She has had a few courses of antibiotics for the treatment of intercurrent infections. While on treatment with a third-generation cephalosporin, she develops a fever, abdominal cramps, and frequent watery diarrhea. Stool is positive for occult blood; microscopy shows an abundance of leukocytes.
What test will produce the fastest and most reliable diagnosis?
1 Culture for C. difficile
2 Detection of C. difficile antigen
3 Enzyme immunoassay for toxin A only
4 Tissue culture cytotoxic assay
5 Enzyme immunoassay for toxins A and B
Enzyme immunoassay for toxins A and B
Enzyme immunoassay for both A and B toxins of C. difficile is a rapid test and helps to verify the diagnosis within 2 hours. Testing should be done without delay after the collection of the sample because the toxin is very unstable. False-negative results occur when specimens are not promptly tested or kept refrigerated until testing can be done.
C. difficile is a Gram-positive, anaerobic, spore-bearing bacillus; it is resistant to many antibiotics. Following prolonged usage of antibiotics (e.g., clindamycin and β-lactams, especially third-generation cephalosporins), the alteration occurring in the intestinal flora of the host leads to a proliferation of the bacterium with production of toxins.
Infection by C. difficile may result in asymptomatic carriage or Clostridioides difficile associated disease (CDAD). The symptoms of CDAD range from mild antibiotic-associated diarrhea to colitis, pseudomembranous colitis, or fulminant colitis. The risk factors include exposure to broad-spectrum antibiotics, gastrointestinal surgery, severe underlying illnesses, immunocompromising conditions, advanced age, and prolonged stays in hospitals (which may act as reservoirs of C. difficile spores). Nosocomial infections are much more common than community-acquired infections. Only toxin-producing strains cause CDAD. The extent of the clinical manifestation depends on the virulence of the infecting strain and the immune response of the individual. Persons with low antitoxin A IgG levels manifest a more severe disease. Most of the clinically significant strains produce toxins A and B. Virulent strains producing only 1 of the toxins have also been reported; therefore, testing for both toxins is important for diagnosis. Toxin A is an enterotoxin that disrupts colonic mucosal cell adherence to the colonic basement membrane and damages villous tips. Toxin B enters the cell and exerts a cytotoxic effect. Necrosis of mucosa with accumulation of inflammatory cells and fibrin form the pseudomembrane. The toxins A and B are encoded by 2 genes, tcdA and tcdB, respectively. Together, with 3 additional genes (2 regulatory genes tcdC and tcdD and a porin gene tcdE), they form a pathogenicity locus (PaLoc) that is integrated into the bacterial chromosome. A new epidemic strain of C. difficile, with variations in the toxin genes and the ability for increased production of A and B toxins, was recently reported. Production of a binary toxin (CDT) and increased resistance to fluoroquinolones are other characteristics of this strain. The role of the binary toxin in pathogenesis is not clear. Partial deletion in the tcdC gene, a putative negative regulator for toxins A and B, is detected in this epidemic strain. This variation is thought to be responsible for increased production of toxins A and B, resulting in increased virulence of the strain. This strain of C. difficile has caused several outbreaks of severe disease in North America and Europe.
Oral metronidazole or vancomycin is used for the treatment of CDAD. Vancomycin is generally considered a second-line treatment due to concerns about resistance development. Neither antibiotic eliminates spores of C. difficile. When treatment is stopped, spores can germinate and C. difficile can proliferate; therefore, relapses are not uncommon. Reinfection also can occur.
Assays based on polymerase chain reaction have been developed for detection of tcdA and tcdB genes in stool samples. These assays have been reported to be useful in diagnosing CDAD.
Stool culture for C. difficile is the most sensitive, but false positive results due to non-toxigenic strains can occur. Toxin production is essential for the development of CDAD. As C. difficile is an anaerobe, culture requires a special environment. In addition, 48-96 hours are required before the results are available. Testing for toxin production by the isolate increases the chance for diagnosis, but the drawback is the prolonged turnover time.
Detection of C. difficile antigen can be done with latex agglutination or by immunochromatographic assay. The tests are rapid; however, false positives due to the presence of non-toxigenic C. difficile can occur. The test has to be combined with enzyme immunoassay for A and B toxins in order to verify the diagnosis.
Enzyme immunoassay for toxin A only is rapid and the result is available in 2 hours. As some virulent strains produce only toxin B, it is important to test for both toxins.
Tissue culture cytotoxic assay detects toxin B only. The toxin is detected using a cell culture of human fibroblasts; the test result is obtained in 24-48 hours. This assay is very sensitive and specific, but needs technical expertise and is expensive.
Cell culture assay using C. difficile strain isolated from the patient has been found to enhance the potential for diagnosis, though this is time-consuming.
Case
A 17-year-old girl presented with sore throat and low-grade fever of 4 days duration. She had received all childhood immunizations. On examination, a grayish white patch was observed on her pharynx. Throat swabs were collected for microbiological examination. Gram stain of throat swab smears showed predominance of Gram-positive beaded bacilli. Smears stained by Albert’s stain showed long slender bacilli with green colored body and deep blue prominent granules. The bacteria were observed to have cuneiform arrangement. Throat swab cultures were done on Loeffler’s serum medium, sheep blood agar, and Hoyle’s tellurite medium. Bacteria showing the same morphology as in the throat swab smears grew on Loeffler’s serum medium, producing small disc-like colonies after 12 hours incubation at 37°C. On blood agar, small beta hemolytic colonies were seen after 24 hours incubation and on tellurite medium, small grayish black colonies appeared after 48 hours incubation. The isolated bacterium was catalase positive, urease negative, pyrazinamidase negative, fermented glucose and maltose with acid production, and reduced nitrate to nitrite. It was susceptible to many antibiotics including penicillin and erythromycin and gave positive Elek’s test.
The pharyngeal infection of the girl is most likely to be due to what bacteria?
1 Corynebacterium xerosis
2 Corynebacterium diphtheriae
3 Corynebacterium pseudotuberculosis
4 Corynebacterium jeikeum
5 Corynebacterium pseudodiphtheriticum
Corynebacterium diphtheriae
The pharyngeal lesion of the patient characterized by grayish white patch is suggestive of diphtheritic pseudomembrane, and the morphology and biochemical characteristics of the isolate are suggestive of Corynebacterium diphtheriae. Elek’s test is an in vitro immunoprecipitation test used for detection of diphtheria toxin. A positive test indicates that the isolate is a toxigenic strain.
Among the other options, Corynebacterium pseudotuberculosis is the only species known to acquire the ability to produce diphtheria toxin. It is a primary pathogen of animals and can be differentiated by a positive urease test and negative nitrate reduction. Corynebacterium xerosis and Corynebacterium pseudodiphtheriticum are non-toxigenic and generally non-pathogenic. Production of pyrazinamidase (pyrazine carboxyl amidase) is one of the phenotypic features used for identification of corynebacterium species. Most non-toxigenic corynebacteria produce pyrazinamidase. Corynebacterium diphtheriae does not produce this enzyme. C. xerosis and C. pseudodiphtheriticum strains are usually pyrazinamidase positive. Susceptibility to antibiotics helps to exclude Corynebacterium jeikeium, which is a multiresistant corynebacterium.
Pharyngeal diphtheria is one of the common clinical manifestations of infection by C. diphtheriae and grayish white pseudomembrane formation is characteristic of the disease. Often marked adenitis, causing swelling of the neck (bull neck appearance), is seen. The most important virulence factor of the organism is its exotoxin. Serious systemic complications like asphyxia due to mechanical obstruction of the respiratory passage by the pseudomembrane and severe toxemia leading to myocarditis and post-diphtheritic paralysis may occur. Once diphtheria is diagnosed clinically, diphtheria antitoxin should be administered to the patient without waiting for the microbiology report. Antibiotics are given to eliminate the organisms. In a patient, clinically diagnosed with pharyngeal diphtheria, culture for the organisms is done to confirm the diagnosis and to exclude staphylococcal and streptococcal pharyngitis and throat lesions associated with other conditions like infectious mononucleosis. Cultural isolation also helps to differentiate the isolate from morphologically resembling commensal corynebacteria and to test for its toxigenicity.
Other clinical forms of diphtheria include laryngeal, nasal, otitic, conjunctival, and cutaneous. Diphtheria was mainly considered a childhood disease and after the introduction of immunization, the incidence of the disease was found considerably reduced in developed countries, and to some extent in developing countries. The prolonged and extensive epidemic of diphtheria by toxigenic diphtheria bacilli in the 1990s in the Soviet Union indicated the dramatic return of the disease and need for strict adherence to immunization policies. Recent outbreaks reported the occurrence of a majority of cases in adolescents and adults. Those who had completed childhood immunization against diphtheria also were found susceptible because of waning immunity. Sporadic cases of diphtheria in adults have been reported in the US as well. Based on these observations, the CDC has recommended booster doses of vaccines containing diphtheria toxoid for all persons above 7 years of age every 10 years in order to maintain adequate immunity among all age groups.
The exotoxin of C. diphtheriae is encoded by tox gene and this gene is conferred by lysogenization by one of the corynebacteriophages, especially the beta phage. The regulation of tox gene expression is mediated by DtxR gene on the bacterial chromosome and is influenced by the iron content in the medium.
Decreased iron concentration inhibits the regulator gene DtxR and leads to increased toxin production. The toxin can be detected by various methods. In vitro Elek’s immunoprecipitation test is the conventional method. PCR-based methods for detection of the tox gene in C. diphtheriae strains and in clinical specimens have been developed and are in use. An immunochromatic strip assay for rapid detection of diphtheria toxin and an enzyme-linked immunosorbant assay (ELISA) test have also been reported to be useful.
Immunization by diphtheria toxoid induces protective levels of antitoxin in circulation, but it has no antibacterial activity. Non-toxigenic strains of C. diphtheriae also are found to be pathogenic. Infections include pharyngitis and also invasive diseases like endocarditis and septic arthritis. Such infections have been more frequently reported in previously immunized individuals. Invasive infections are reported to be often caused by certain invasive clones and are seen in injection drug users and homeless alcoholics, transmission being facilitated by crowding and unhygienic living conditions. The pathogenesis of non-toxigenic C. diphtheriae is not clearly understood.
Corynebacterium xerosis is a human commensal corynebacterium found in the conjunctival sac. It is pyrazinamidase positive, unlike C.diphtheriae, and is not known to produce toxin. Infections caused are very rare, mainly affecting immunocompromised patients. It has been isolated from patients with endocarditis with predisposing cardiovascular lesions.
Corynebacterium pseudotuberculosis is of veterinary importance. It produces caseous lymphadenitis in horses, sheep, and goats. Cases of human lymphadenitis due to this organism mostly associated with occupational exposure have been reported, though very rarely. C. pseudotuberculosis is urease positive, pyrazinamidase negative.
Corynebacterium jeikeium is an opportunistic pathogen causing acute invasive infections like endocarditis in immunosuppressed individuals. The infections are associated with high mortality, as the bacterium shows high resistance to most antibiotics, responding only to vancomycin.
Corynebacterium pseudodiphtheriticum (C. hofmannii) is a commensal of the throat. Though it morphologically resembles C. diphtheriae, it can be differentiated by biochemical tests. It does not ferment sugars and is urease and pyrazinamidase positive. It is reported to cause endocarditis in patients with prosthetic valve replacement and is an uncommon cause of respiratory infections and community-acquired pneumonia.
Case
A 72-year-old man was brought to the hospital with sudden onset of rigors and high fever (38.8°C), vomiting, and confused mental status. The patient had a history of fever and diarrhea about 3 weeks earlier from which he recovered without any specific treatment. While being examined, he developed generalized seizures. Lumbar puncture was done. CSF was turbid. CSF and blood samples were sent for microbiological investigations and the patient was placed on intravenous ceftriaxone as empirical therapy.
The CSF total WBC count was 1000/cmm. Differential count showed 63% mononuclear cells with predominance of lymphocytes. On Gram stain done with centrifuged deposit of CSF, very few Gram-positive coccobacilli could be observed. Kinyoun’s and modified acid-fast stains showed nonacid-fast coccobacilli. After 48 hours of incubation, minute round smooth translucent beta hemolytic colonies of short Gram-positive bacilli appeared on aerobic sheep blood agar. The bacteria showed tumbling motility when grown in broth at 25°C and were nonmotile at 37°C. The isolate was catalase positive and fermented sugars with production of acid only. Antibiotic susceptibility pattern showed sensitivity to penicillin, trimethoprim/sulphamethoxazole, vancomycin, and aminoglycosides, as well as resistance to quinolones and third generation cephalosporins. Gram-positive bacillus with same characteristics was grown from blood culture samples. Based on the microbiology report, the patient was placed on a prolonged course of combination therapy with ampicillin and gentamicin.
What organism is the cause of meningitis in this patient?
1 Flavobacterium meningosepticum
2 Listeria monocytogenes
3 Corynebacterium jeikeium
4 Rhodococcus equi
5 Erysipelothrix rhusiopathiae
Listeria monocytogenes
The mononuclear predominance and presence of gram-positive coccobacilli in the CSF are in favor of Listeria monocytogenes infection. Morphology in Gram-stained smear and cultural characteristics of the isolate from the clinical samples are typical of Listeria though they are not sufficient to differentiate from corynebacterium, which the organism closely resembles. The tumbling motility shown by the bacterium when grown at 25°C and its absence at 37°C is highly characteristic of L. monocytogenes and is an important differentiating feature. The antibiotic sensitivity pattern is also in accordance with that of L. monocytogenes, which is known to be resistant to third generation cephalosporins.
Listeria monocytogenes causes severe invasive infections such as meningitis and meningoencephalitis in neonates, pregnant women, elderly, transplant patients, and others with impaired cell-mediated immunity. Listeria can also cause infections in normal individuals. Infection is usually foodborne and is transmitted by consumption of unpasteurized dairy products and uncooked meat and vegetables. Outbreaks of listeriosis associated with contaminated food have occurred, though rarely. The bacterium possesses the ability to cross intestinal, placental, and blood-brain barriers leading to gastroenteritis, maternofetal infections, meningitis, and meningoencephalitis. For invasive illness, the incubation period is 20-30 days and for gastroenteritis about 20 hours. Important sero varieties, which are associated with human infections, are 1a, 1b, and 4b. Some of the factors, which attribute to the virulence of Listeria monocytogenes, are its cell wall surface proteins called Internalins, hemolysin (listeriolysin O), phospholipase enzymes, and ActA protein that are responsible for the actin-based intracellular motility of the bacterium. Prolonged treatment (2-3 weeks) with high doses of antibiotics is required for invasive infections because of the intracellular habitat of the bacterium. Combined therapy with ampicillin and gentamicin is considered the best option. Trimethoprim-sulfamethoxazole is used in those who cannot take penicillin.
Flavobacterium meningosepticum (Chrysobacterium meningosepticum) is a nonmotile Gram-negative bacillus, which often produces yellow-pigmented colonies on blood agar medium. It can cause meningitis and sepsis, especially in the newborns and in adults with underlying illness. It is often associated with nosocomial infections such as ventilator-associated pneumonia. It may also cause endocarditis. In the hospital environment, the bacterium exists in water systems on wet surfaces of medical tools and equipment. Vancomycin has been successfully used for the treatment of meningitis caused by this Gram-negative bacterium. Newer quinolones garenoxacin, gatifloxacin, and levofloxacin have been reported to be most effective antibiotics for treating infections by Chrysobacterium meningosepticum. The bacterium is known to be resistant to aminoglycosides, tetracyclines, erythromycin, clindamyciin, teicoplanin, and most beta-lactams, including carbapenems. Chromosomal genes that encode for production of metallo-betalactamases have been identified in this bacterium. Dissemination of these genes to other Gram-negative bacteria of greater clinical significance can pose a problem.
Corynebacterium jeikeium may resemble listeria on Gram stain, but is nonmotile and non-hemolytic. It can cause severe infections such as endocarditis in immunocompromised individuals and is known to be resistant to most of the commonly used antimicrobials. Vancomycin is the antibiotic of choice for treatment. Macrolide resistance of C. jeikeium has been observed as part of its multidrug resistance. Genetic studies have shown that macrolide resistance is conferred by erm (X) cj gene, which is integrated within the chromosome of this bacterium. It is suggested that C. jeikeium may be an important reservoir of drug resistant genes and the presence of this organism in the hospital environment can be a cause for concern.
Rhodococcus equi (Corynebacterium equi) is a Gram-positive pleomorphic coccobacillus. It is weakly acid fast when stained with modified acid-fast stain. On blood agar, it produces non-hemolytic, mucoid colonies with salmon pink pigmentation. It is primarily a pathogen of animals and produces severe pneumonia in foals. It is an intracellular pathogen and is an important cause of AIDS-associated pneumonia in humans. R. equi is also known to cause infections in persons having other conditions with impaired cell-mediated immunity, such as organ transplantation and malignancy. In immunocompetent persons, R. equi infection is extremely rare. Virulence-associated antigens (VapA and VapB) and virulence plasmids of R. equi have been discovered. The majority of R. equi isolates from patients with AIDS have been shown to harbor virulence plasmids and either of the 2 virulence-associated antigens. R. equi is resistant to penicillins and cephalosporins and sensitive to vancomycin, rifampin, aminoglycosides, erythromycin, and imipenem.
Erysipelothrix rhusiopathiae is a Gram-positive bacillus that may appear singly, in short chains, or as long branching filaments. It grows slowly on blood agar medium producing small transparent glistening colonies either non-hemolytic or alpha hemolytic. It is catalase negative, highly susceptible to Penicillin G, and intrinsically resistant to vancomycin. It causes erysipelas in swine. In humans, the bacterium produces erysipeloid, a localized cutaneous lesion. The lesion follows inoculation of the organisms at the site of a cut or abrasion and usually occurs on the hands or fingers of persons handling animals, fish, or animal products. It is painful, edematous, and erythematous. Often the local lymph glands are involved. Generalized cutaneous lesions may occur due to spread from the initial site of infection. In rare cases, the bacterium causes septicemia associated with endocarditis. Penicillin G is the drug of choice for severe infections. The mortality rate of septicemia is high because vancomycin is often used as empirical therapy; therefore, early diagnosis is important. Polymerase chain reaction (PCR) assays have been recently developed for detection and identification of E. rhusiopathiae.
A 10-year-old boy is brought to the emergency room by his mother. He has a 4-day history of malaise, sore throat, anorexia, and low-grade fever. On PE, pharyngeal inflammation and a gray pseudomembrane covering the tonsils are visualized, which bleeds on trying to remove it. Culture shows black colonies and smear shows club-shaped gram-positive rods. How does the toxin produced by this bacteria act?
1 It decreases glycine and gamma-amino butyric acid by protein cleavage
2 It increases cAMP by ADP-ribosylation of Gi-alfa protein
3 It increases cAMP by ADP-ribosylation of the Gs-alfa protein
4 It inhibits the protein synthesis by adding ADP-ribose to the elongation factor 2
5 It stimulates adenylate cyclase by ADP ribosylation of GTP-binding protein
It inhibits the protein synthesis by adding ADP-ribose to the elongation factor 2
Eukaryotic cells need the elongation factor-2 for the translocation in the protein synthesis process. This can be inactivated by ADP-ribosylation by microorganisms such as pseudomonas and diphtheria.
The Escherichia coli produce a toxin that stimulates adenylate cyclase by ADP ribosylation of GTP-binding protein.
The Bordetella pertussis toxin increases cAMP by ADP-ribosylation of Gi.
The Vibrio cholera toxin increases cAMP by ADP-ribosylation of the Gs-alfa protein.
The Clostridium tetani toxin decreases glycine and gamma-amino butyric acid in the synaptic cleft. These are inhibitor factors. It appears to act by cleavage of the protein synaptobrevin II.
The capsule of Bacillus anthracis is made of
1 D-glutamic acid polypeptide
2 Diaminopimelic acid
3 Dipicolonic acid
4 Muramic acid
D-glutamic acid polypeptide
The prime B.anthracis virulence factors are its capsule and production of exotoxins. The capsule is a D-glutamic acid polypeptide of a single antigenic specificity. It has antiphagocytic properties and is required for full virulence.
Diphtheria toxin acts by
1 Inhibiting protein synthesis
2 Blocking mRNA binding
3 Producing edema
4 Increasing translation
Inhibiting protein synthesis
Diphtheria toxin acts by inhibiting protein synthesis. It contains both an active subunit A, which catalyzes the toxic activity, and a subunit B, which mediates receptor binding and membrane translocation. Separation of the two subunits is required for full activity of the A subunit on its target protein elongation factor 2 (EF-2), which transfers polypeptidyl transfer RNA from acceptor to donor sites on the ribosome of the host cell. The specific action of the A subunit is to catalyze the transfer of the adenosine ribose phosphate portion of the nicotinamide adenine dinucleotide (NAD) to EF-2, an enzymatic reaction called ADP-ribosylation. Covalent attachment of the ADP-ribosyl groups occurs at an unusual derivative of histidine called dipthamide. This inactivates EF-2 and shuts off protein synthesis.
Corynebacteria are
1 Small, pleomorphic gram-negative bacilli
2 Motile
3 Non-spore formers
4 Devoid of mycolic acid
5 Strict anaerobes
Non-spore formers
Corynebacteria are non-motile, non-spore forming, small, pleomorphic gram-positive bacilli. The cell wall consists of short-chain mycolic acids, meso-diaminopimelic acid, and arabino-galactan polymers. These organisms are aerobes or facultative anaerobes.
Corynebacteria are
1 Motile
2 Catalase positive
3 Spore formers
4 Do not ferment carbohydrates
5 Strict anaerobes
Catalase positive
Corynebacteria are non motile, catalase positive, non-spore formers. They are aerobic or facultatively anaerobic and ferment carbohydrates, producing lactic acid.
DPT vaccine contains
1 Toxoid for diphtheria and tetanus and heat killed organisms for whooping cough
2 Toxoid for tetanus and heat killed organisms for diphtheria and whooping cough
3 Toxoid for whooping cough and tetanus and heat killed organisms for diphtheria
4 Toxoid for diphtheria, tetanus and whooping cough
5 Heat killed organisms for diphtheria, tetanus and whooping cough
Toxoid for diphtheria and tetanus and heat killed organisms for whooping cough
Diphtheria, tetanus and whooping cough can be prevented by active immunization with DPT vaccine during childhood and booster immunization every 10 years throughout life.
DPT vaccine contains toxoid for diphtheria and tetanus( it is nontoxic and immunogenic and prepared by formalin treatment of toxin) and heat killed Bordetella pertusis, causative agent of whooping cough.
The test that determines immunity to diphtheria by measuring the presence of neutralizing antibodies in an individual’s circulating system is called
1 Elek test
2 Dick test
3 Schick test
4 Schultz-Charlton test
5 Frei test
Schick test
Schick test: This test is done by injecting diphtheria toxin intradermally. No skin reaction indicates presence of neutralizing antibodies. However, localized edema with necrosis suggests absence of neutralizing antibodies to diphtheria toxin.
Elek test: An immunodiffusion test used for in vitro detection of diphtheria toxin in an isolate.
Dick test: Scarlet fever exotoxin injected intradermally produces localized erythema at 24hr in an individual susceptible to scarlet fever.
Schultz-Charlton test: When an antitoxin to the scarlet fever is injected in to an area of rash and the rash clears, it indicates that patients has scarlet fever and if it doesn’t, then rash is due to other causes.
Frei test: Skin test for lymphogranuloma venereum
The most common cause of diphtheroid prosthetic valve endocarditis in adults is
1 Corynebacterium ulcerans
2 Corynebacterium striatum
3 Corynebacterium urealyticum
4 Corynebacterium jeikeium
5 Corynebacterium kutcheri
Corynebacterium jeikeium
Corynebacterium jeikeium is part of normal skin flora and causes infection in immunocompromised patients or patients undergone invasive procedure. The presence of catheters or prosthetic devices also contributes to infection with Corynebacterium jeikeium. This organism shows high degree of antimicrobial resistance.
Corynebacterium striatum is found in nasopharynx and rarely causes infection.
Corynebacterium urealyticum is associated with urinary tract infection.
Corynebacterium ulcerans is a veterinary pathogen that can be isolated from skin ulcers or exudative pharyngitis.
Corynebacterium kutcheri is an animal pathogen.
Cognitive Level: Understand
Listeria monocytogenes is
1 A facultative, intracellular pathogen
2 Gram-positive cocci
3 Spore former
4 Non motile
5 α-hemolytic
A facultative, intracellular pathogen
Listeria monocytogenes is motile, non-sporulating, short gram-positive rod. It produces β-hemolysis on blood agar and has tumbling motility in broth incubated at room temperature. It is an intracellular pathogen and is capable of growth in macrophages, epithelial cells and fibroblasts. Listeria monocytogenes is catalase positive like Corynebacterium diptheriae.
Listeria monocytogenes is
1 Motile, α-hemolytic, and catalase positive
2 Non-motile, α-hemolytic, and catalase positive
3 Non-motile, β-hemolytic, and catalase negative
4 Non-motile, α-hemolytic, and catalase negative
5 Motile, β-hemolytic, and catalase positive
Motile, β-hemolytic, and catalase positive
Listeria monocytogenes is characterized by umbrella-shaped motility at room temperature. It gives β-hemolysis on blood agar and resembles group B β-hemolytic streptococci.
Listeria monocytogenes is motile and gives positive catalase test.
Group B streptococci are non-motile, β-hemolytic, and catalase negative.
Listeria monocytogenes and Streptococci agalactiae are β-hemolytic on sheep blood agar. The isolate can be identified as Listeria monocytogenes if it gives
1 Positive catalase test, positive motility and positive hippurate hydrolysis
2 Negative catalase test, negative motility and negative hippurate hydrolysis
3 Negative catalase test, negative motility and positive hippurate hydrolysis
4 Positive catalase test, positive motility and negative hippurate hydrolysis
5 Positive catalase test, negative motility, and positive hippurate hydrolysis
Positive catalase test, positive motility and negative hippurate hydrolysis
Listeria monocytogenes and Streptococci agalactiae are β-hemolytic on sheep blood agar and are positive for esculin hydrolysis and give positive CAMP reaction.
Listeria monocytogenes gives positive catalase test and negative hippurate hydrolysis test. It gives characteristic umbrella-shaped motility.
Streptococci agalactiae is non motile gram positive cocci that gives negative catalase test and positive hippurate hydrolysis tes
The causative agent of erysipeloid is
1 Streptococci pyogenes
2 Staphylococci aureus
3 Staphylococci epidermidis
4 Listeria monocytogenes
5 Erysipelothrix rhusiopathie
Erysipelothrix rhusiopathie
rysipelothrix rhusiopathie causes erysipeloid which is a localized skin infection that resembles streptococcal erysipelas. The lesions appear on the hands or fingers because the organisms are inoculated through work activities. The area is accompanied by a lesion that is a sharply defined, slightly elevated, with purplish red zone that spreads peripherally as discoloration of the central area fades.
Clinical manifestation includes low-grade fever, arthralgia, lymphangitis, and lymphadenopathy.
Erysipelas is an infection of skin and subcutaneous tissues most often seen in elderly patients and is caused by Streptococci pyogenes.
Erysipelothrix rhusiopathiae is
1 Gram positive rod
2 Motile
3 β-hemolytic on blood agar
4 Catalase positive
5 Spore former
Gram positive rod
Erysipelothrix rhusiopathiae is a non motile, non-spore forming, thin gram positive rod that may form long filaments. The colonies on blood agar are non hemolytic or α-hemolytic and appear as pinpoint after 24 hours incubation.
Erysipelothrix rhusiopathiae gives a negative catalase test.
