Gram-Positive Bacteria (Staphylococcus, Streptococcus, Clostridium, Diphtheria) MCQs

Microbiology · 94 free questions with answers & explanations.

1. A neonate develops widespread superficial blistering and exfoliation of skin in large sheets. The causative Staphylococcus aureus strain is most commonly from phage group II. The toxin responsible acts by:
2. A 20-year-old develops acute rheumatic fever 3 weeks after untreated streptococcal pharyngitis. The mechanism of cardiac damage in rheumatic fever is best explained by:
3. A post-abdominal surgery patient develops sudden onset severe watery diarrhea with pseudomembranes seen on colonoscopy, following a course of broad-spectrum antibiotics. The toxin of the causative organism acts by:
4. A 7-year-old unvaccinated child develops a greyish-white adherent pseudomembrane in the throat that bleeds on attempted removal. The organism produces a toxin encoded by a bacteriophage that acts by:
5. A wound infection in a diabetic patient progresses to crepitant myonecrosis. Gram stain shows large Gram-positive rods. The Nagler reaction on egg-yolk agar (inhibited by specific antitoxin) is positive. The major virulence toxin is:
6. Methicillin-resistant Staphylococcus aureus (MRSA) resistance to all beta-lactam antibiotics is mediated by expression of a modified penicillin-binding protein. What is the specific genetic element responsible for this resistance?
7. Staphylococcal toxic shock syndrome toxin-1 (TSST-1) acts as a superantigen. Its unique mechanism of T-cell activation, compared to conventional antigens, is:
8. Clostridium difficile–associated diarrhea is caused by two main toxins. Toxin B (cytotoxin) differs from Toxin A (enterotoxin) in its primary mechanism of action in that Toxin B:
9. A 5-year-old unvaccinated child presents with a greyish-white pseudomembrane in the throat. Corynebacterium diphtheriae toxin is produced only by lysogenic strains carrying which bacteriophage?
10. A patient develops descending flaccid paralysis beginning with diplopia and dysphagia after consuming home-canned vegetables. Which step in neurotransmitter release is specifically blocked by Clostridium botulinum toxin?
11. Staphylococcus aureus TSST-1 causes toxic shock syndrome through a superantigen mechanism. This differs from conventional antigen presentation because TSST-1:
12. A neonate develops necrotizing enterocolitis. Blood culture grows S. agalactiae (Group B Streptococcus). The virulence factor most critical for this organism to resist phagocytosis in the neonatal bloodstream is:
13. Clostridium difficile colitis is mediated by two exotoxins. The specific mechanism by which Toxin B (TcdB) causes mucosal cell death is:
14. Corynebacterium diphtheriae produces a toxin encoded by a bacteriophage. The specific biochemical target of diphtheria toxin that halts protein synthesis is:
15. A postoperative patient develops gas gangrene. Culture grows Clostridium perfringens. The alpha toxin responsible for membrane disruption is classified as a:
16. A 70-year-old hospitalised patient develops severe watery diarrhoea with 10 bowel movements per day, abdominal pain, and fever on day 8 of clindamycin therapy for aspiration pneumonia. Sigmoidoscopy reveals yellow-white plaques on the colon mucosa. Which virulence factor directly causes the mucosal damage in this condition?
17. Methicillin-resistant Staphylococcus aureus (MRSA) resistance to all beta-lactam antibiotics is conferred by which specific molecular mechanism?
18. A 3-day-old neonate develops pneumonia and empyema. Blood culture grows catalase-positive, CAMP test positive, beta-haemolytic Gram-positive cocci that are resistant to penicillin. The most likely organism and recommended therapy are:
19. The diphtheria toxin produced by lysogenic Corynebacterium diphtheriae is encoded by the tox gene located on a bacteriophage. The enzymatic mechanism of toxin-mediated cell death involves:
20. A 28-year-old woman with recurrent vulvovaginal candidiasis and poor wound healing is found to have absent circulating T lymphocytes on flow cytometry. ADA enzyme activity is undetectable in erythrocyte lysate. The underlying pathogenesis involves accumulation of which toxic metabolite in lymphocytes?
21. Staphylococcus aureus producing toxic shock syndrome toxin-1 (TSST-1) causes fever, rash, and multi-organ dysfunction. TSST-1 acts as a superantigen. Which immunological mechanism explains its ability to cause massive cytokine release?
22. Clostridium tetani produces tetanospasmin which causes spastic paralysis. What is the precise molecular mechanism of action of tetanospasmin at the level of inhibitory interneurons?
23. A blood culture isolate shows beta-hemolysis, Lancefield group B antigen, positive CAMP test, and hydrolysis of hippurate. Which virulence factor of this organism is most important for neonatal meningitis?
24. Diphtheria toxin exerts its pathological effects via ADP-ribosylation. Identification of the specific molecular target helps explain the cardiac and neural toxicity. Which is the correct target?
25. Laboratory diagnosis of Clostridium difficile infection (CDI) in a patient on antibiotics with pseudomembranous colitis should include detection of toxins. What is the GOLD STANDARD diagnostic method and which toxins should be specifically detected?
26. A clinical isolate of Staphylococcus aureus is identified with a MIC of 4 µg/mL for oxacillin. The resistance mechanism in this organism involves a gene encoding a modified penicillin-binding protein. Which PBP and gene are responsible?
27. A throat swab from a child with membranous pharyngitis grows grey-black colonies on tellurite medium. Toxigenicity is confirmed using the Elek gel precipitation test. The diphtheria toxin gene (tox) is carried by:
28. A post-operative patient develops severe watery diarrhoea after a course of clindamycin. Stool toxin assay is positive for toxin A and toxin B. Regarding the pathogenesis of Clostridioides difficile, toxin B (cytotoxin):
29. Streptococcus pyogenes (GAS) M protein is the primary virulence factor enabling immune evasion. Its antiphagocytic mechanism primarily involves:
30. CAMP test is used to definitively identify which organism, and what is the biochemical basis of the positive result?
31. S. aureus methicillin resistance is mediated by the mecA gene encoding PBP2a. A clinical isolate of MRSA is being tested for sensitivity. The minimum inhibitory concentration (MIC) of vancomycin is 2 µg/mL. According to CLSI breakpoints, this isolate would be classified as:
32. A 55-year-old man with prosthetic valve endocarditis grows coagulase-negative Staphylococcus (CoNS) from three separate blood culture sets. The most virulent species associated with prosthetic device infections and the one MOST likely to be β-lactam resistant is:
33. A 3-year-old child in Tamil Nadu develops sudden high fever, sore throat, and a fine, diffuse erythematous rash with circumoral pallor and a 'sandpaper' texture. The tongue has a white coating with enlarged red papillae. What is the virulence mechanism of the causative toxin?
34. Streptococcus agalactiae (Group B Streptococcus) causing neonatal meningitis. The primary virulence factor enabling GBS to colonise neonatal meninges and evade complement killing is:
35. Enterococcus faecium is isolated from a blood culture of a patient with a central venous catheter. Sensitivity testing shows high-level resistance to ampicillin (MIC >16 µg/mL) and vancomycin (VRE, MIC >32 µg/mL). The most appropriate antibiotic for this VRE bacteraemia is:
36. A 55-year-old diabetic man develops a painful indurated area on his perineum that rapidly progresses to crepitant necrosis over 24 hours with systemic sepsis. Gram stain shows mixed gram-positive cocci and gram-negative rods. What is the virulence mechanism of the streptococcal component (Group A Streptococcus) that most critically drives this necrotising fasciitis phenotype?
37. Staphylococcus epidermidis biofilm on a central venous catheter is being studied. Which component of the biofilm formation sequence does the ica operon (intercellular adhesin locus) encode for?
38. A neonate develops extensive superficial bullous skin lesions that rupture easily, leaving moist raw areas. The mother had a Staphylococcus aureus skin infection. Which toxin is responsible, and what is its molecular target in the skin?
39. Enterococcus faecalis is isolated from blood cultures of a patient with subacute infective endocarditis. Susceptibility testing shows resistance to ampicillin and high-level aminoglycoside resistance (HLAR). The isolate tests vancomycin-sensitive. What is the MOST appropriate antibiotic regimen?
40. In the Elek's gel precipitation test used for diphtheria toxin detection, what is the principle and what does a positive result look like?
41. A patient with a prosthetic heart valve develops subacute endocarditis caused by a coagulase-negative Staphylococcus. The organism is slime-producing, novobiocin-sensitive, and urease-negative. Which species is most likely?
42. An infant develops sudden onset of generalised blistering with a positive Nikolsky sign. S. aureus is isolated from a distant skin site (not the blisters). The toxin responsible acts by cleaving which skin protein?
43. A throat swab from a child with sore throat grows beta-haemolytic colonies on blood agar. Bacitracin sensitivity testing shows sensitivity (>10 mm inhibition zone). This feature identifies the organism as:
44. MRSA strains are resistant to all beta-lactam antibiotics. The mechanism of this resistance is best described as:
45. Diphtheria toxin causes its lethal effect by inactivating which cellular enzyme/factor?
46. A neonate develops scalded skin syndrome (Ritter disease) within 48 hours of birth. The causative agent produces an exfoliative toxin (ET). Which specific property of ET explains the blister formation restricted to the superficial epidermis (stratum granulosum) WITHOUT causing dermal involvement?
47. A 55-year-old diabetic man develops necrotizing fasciitis of the left lower limb after a minor abrasion. Blood cultures grow Streptococcus pyogenes (Group A Streptococcus). He develops high fever, shock, and multi-organ failure — consistent with streptococcal toxic shock syndrome (STSS). The virulence factor primarily mediating the toxic shock is:
48. A 25-year-old IV drug user presents with fever, rigors, and a new pansystolic murmur at the tricuspid area. Blood cultures taken on two separate occasions grow Staphylococcus aureus. Transthoracic echocardiography confirms large tricuspid vegetations. Which feature distinguishes S. aureus endocarditis bacteremia from transient bacteremia in terms of clinical management?
49. Diphtheria toxin mechanism involves ADP-ribosylation of Elongation Factor-2 (EF-2), halting protein synthesis. Which fragment of the toxin is responsible for ADP-ribosyltransferase activity, and what is the unique amino acid target?
50. A 40-year-old post-appendectomy patient develops abdominal pain, diarrhea (up to 15 watery stools/day), and fever on day 8 of ceftriaxone therapy. Sigmoidoscopy shows yellowish plaques on colonic mucosa. Stool testing for Clostridioides difficile toxin (A/B) by EIA is negative. The NEXT BEST diagnostic step is:
51. A neonate develops rapidly progressing cellulitis with bullae formation. Cultures grow Staphylococcus aureus that produces a serine protease cleaving desmoglein-1. The syndrome is:
52. A 60-year-old develops infective endocarditis after dental extraction. Blood cultures grow alpha-hemolytic streptococci that are optochin-resistant and bile-insoluble. The most likely organism is:
53. Clostridium difficile is confirmed by toxin EIA in a hospitalized patient who has received broad-spectrum antibiotics. He has 10 watery stools per day but is hemodynamically stable. What is the first-line treatment according to current IDSA guidelines?
54. A patient presents with ascending flaccid paralysis after consuming home-canned vegetables. Electromyography shows decremental response at low-frequency stimulation. The toxin acts by:
55. On throat swab culture for suspected diphtheria, grey-white pseudomembrane colonies on Loeffler's serum slope show metachromatic granules (volutin) with Albert's stain. To confirm toxin production, the most specific in vitro test is:
56. An ICU patient develops a wound infection. Swab grows Staphylococcus aureus. Disc diffusion shows resistance to oxacillin; the isolate also tests positive by cefoxitin disc test. Real-time PCR of the isolate is positive for mecA gene. The mechanism underlying cefoxitin disc resistance is best explained by:
57. A microbiologist notices that a throat swab isolate of Streptococcus pyogenes is resistant to bacitracin on disc testing. Which additional test should confirm the provisional identification of this organism?
58. A 60-year-old diabetic with leg wound infection has an anaerobic culture growing Clostridium perfringens. The pathogenesis of myonecrosis (gas gangrene) is primarily driven by which toxin, and what is its enzymatic mechanism?
59. An Elek test is performed on a throat swab isolate suspected to be Corynebacterium diphtheriae. A precipitin line forms at 45° to the central antitoxin-impregnated filter paper strip. What does this finding indicate, and on which medium is colony morphology first assessed?
60. A 16-year-old girl develops acute glomerulonephritis two weeks after a skin infection with Group A Streptococcus. Her throat culture is negative. ASO titres are low but anti-DNase B titres are elevated. What does this pattern indicate?
61. A 4-day-old neonate in the NICU develops scalded skin syndrome (SSSS). Blood culture is negative. Which virulence factor is responsible and from where is the responsible organism likely colonised?
62. A patient on clindamycin for an anaerobic infection develops watery diarrhoea on day 10 of therapy. Stool enzyme immunoassay detects both toxin A and toxin B of Clostridioides difficile. What is the mechanism by which toxin B (TcdB) causes colonocyte damage?
63. Tellurite agar is used as a selective and differential medium in the isolation of Corynebacterium diphtheriae. A throat swab from a suspected diphtheria case grows black colonies on tellurite agar. Albert's stain of the organism shows metachromatic granules (volutin granules). Which staining pattern is characteristic?
64. A neonate develops bullous skin lesions at 5 days of age. Nikolsky's sign is positive. The lesions are caused by an exfoliative toxin (ET) that cleaves desmoglein-1. The most likely causative organism is:
65. A 40-year-old develops hemolytic anemia, hemoglobinuria, and acute kidney injury following a clostridial septicemia. The toxin directly responsible for massive intravascular hemolysis is:
66. A patient develops toxic shock syndrome after a soft tissue infection. Blood culture grows Streptococcus pyogenes. The superantigenic toxin responsible for massive T-cell activation in streptococcal TSS is:
67. On the CAMP test, a hemolytic organism produces enhanced (arrow-shaped) hemolysis when streaked perpendicular to a beta-hemolytic Staphylococcus aureus streak. This organism most likely is:
68. A throat swab from a patient with a grey pseudomembrane grows colonies on tellurite medium showing black metallic colonies. Elek's test is performed. This test detects:
69. A 55-year-old diabetic man has a rapidly spreading necrotizing skin infection. Gram stain of wound exudate shows Gram-positive cocci in clusters. The isolate is coagulase-positive and is resistant to oxacillin by disc diffusion (zone of inhibition < 10 mm for cefoxitin disc). The MIC for vancomycin is 1 µg/mL. Which term best categorizes this isolate?
70. A 4-year-old child presents with fever, drooling, and 'hot-potato voice.' On examination, there is bulging of the posterior pharyngeal wall. Throat swab Gram stain shows Gram-positive cocci in chains. The organism is beta-hemolytic, bacitracin-sensitive, and PYR-positive. Treatment should be:
71. A patient develops profuse rice-water diarrhea after a picnic. Stool anaerobic culture shows heat-resistant spore-forming Gram-positive rods producing lecithinase (Nagler reaction positive) on egg-yolk agar. Toxin testing on Vero cells shows enterotoxin with cytotoxic activity. The organism is most likely:
72. Tellurite reduction in corynebacteria cultures produces which characteristic appearance on Hoyle's tellurite blood agar, and what does this indicate?
73. Staphylococcal toxic shock syndrome (TSS) is mediated by TSST-1, which acts as a superantigen. The superantigen mechanism that causes the massive cytokine release involves:
74. Blood cultures from a patient with prosthetic valve endocarditis grow a coagulase-negative Staphylococcus. The isolate tests negative with the tube coagulase test and positive with the latex agglutination test for clumping factor. Which species is most likely, and what is its key virulence factor for biofilm formation on prosthetics?
75. A 3-year-old child develops sudden onset fever, sore throat, and a greyish-white membrane firmly adherent to the tonsillar surface extending to the uvula. Attempting to remove the membrane causes bleeding. Gram stain shows club-shaped organisms in Chinese letter arrangement. Which test confirms the toxigenicity of the isolate?
76. A 60-year-old diabetic develops rapidly spreading crepitant wound infection of the lower limb with foul-smelling discharge. X-ray shows gas in soft tissues. Anaerobic culture grows large, box-car shaped Gram-positive rods with subterminal spores that are non-swelling. Double zone of haemolysis is seen on blood agar. Which organism is this?
77. A neonate develops neonatal meningitis on day 3. CSF culture grows a beta-haemolytic Gram-positive coccus in chains. The CAMP test is positive and hippurate hydrolysis is positive. Which serogroup is most pathogenic in neonates and what is the first-line treatment?
78. A post-operative patient develops acute-onset watery diarrhoea on day 10 of broad-spectrum antibiotic therapy. Sigmoidoscopy reveals pseudomembranes. Stool enzyme immunoassay (EIA) tests for toxin A only returns negative. What is the most appropriate next step in diagnosis?
79. A child is admitted with bull-neck, hoarseness, low-grade fever and a grey-green pseudomembrane extending from the tonsil to the soft palate. Loeffler's serum slope culture shows club-shaped bacilli in 'Chinese letter' arrangement. Which laboratory test confirms the toxigenicity of the isolated organism?
80. A blood culture from an immunocompromised patient grows an organism that is catalase-positive, coagulase-negative, novobiocin-resistant, and forms a yellow pigment on nutrient agar. Which Staphylococcus species is most likely, and what is its clinical significance?
81. Lancefield grouping by latex agglutination identifies a beta-haemolytic streptococcus as Group B (Streptococcus agalactiae). In the diagnostic workup of a neonate with early-onset sepsis, which screening test is recommended for maternal colonisation status at 35–37 weeks gestation?
82. Toxic shock syndrome toxin-1 (TSST-1) of Staphylococcus aureus acts as a superantigen. Its mechanism of T cell activation differs from conventional antigen presentation in that it:
83. Streptococcus agalactiae (Group B Streptococcus) can be distinguished from Group A Streptococcus on blood agar by which test?
84. Clostridium difficile produces two major toxins in hospital-associated diarrhoea. Toxin B (cytotoxin) differs from Toxin A (enterotoxin) in which property relevant to clinical diagnostics?
85. The Elek (modified Elek) test for Corynebacterium diphtheriae assesses toxin production. Which component serves as the reference in the test and what does a precipitation line indicate?
86. A 5-year-old child presents with sudden onset fever, a 'strawberry tongue,' and sandpaper-like rash that blanches on pressure, sparing the perioral area. The most likely organism produces a toxin that causes rash by its specific mechanism. What toxin and mechanism is responsible for the rash of scarlet fever?
87. A diabetic patient has a deep wound infection of the right leg with rapidly spreading necrosis, crepitus on palpation, and 'dishwater' foul-smelling exudate. X-ray shows gas in soft tissues. Gram stain of exudate shows large gram-positive bacilli with blunt ends (boxcar appearance), few inflammatory cells, and no spores seen on routine staining. What is the virulence factor primarily responsible for local tissue destruction?
88. Corynebacterium diphtheriae toxin is only produced by strains lysogenised by a specific bacteriophage. The gene for diphtheria toxin is carried on which phage, and what host protein does the toxin ADP-ribosylate to inhibit protein synthesis?
89. A patient undergoing chemotherapy develops neutropenic fever. Blood cultures grow Viridans group Streptococcus. Which antibiotic combination should be used empirically pending susceptibility results?
90. The toxin of Clostridium difficile that is primarily responsible for colonic cell damage and pseudomembranous colitis is:
91. Diphtheria toxin production by Corynebacterium diphtheriae requires infection by a bacteriophage carrying the tox gene. The mechanism by which diphtheria toxin kills mammalian cells is:
92. Streptococcus pneumoniae is the commonest cause of community-acquired pneumonia. On blood agar it shows alpha-haemolysis and is bile-soluble and optochin-sensitive. In cerebrospinal fluid from bacterial meningitis, which rapid bedside test confirms pneumococcal aetiology?
93. A patient admitted for hip fracture repair develops profuse watery diarrhoea after 5 days of cephalosporin treatment. Stool ELISA is positive for Clostridium difficile toxin B. The preferred first-line treatment is:
94. Group B Streptococcus (Streptococcus agalactiae) is a leading cause of neonatal septicaemia and meningitis. Intrapartum prophylaxis is indicated in which scenario?