Gram-Negative Bacteria (E. coli, Salmonella, Shigella, Vibrio, Klebsiella) MCQs

Microbiology · 74 free questions with answers & explanations.

1. A 5-year-old develops bloody diarrhea followed by hemolytic uremic syndrome (microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure). Stool cultures on Sorbitol-MacConkey agar shows colorless (sorbitol non-fermenting) colonies. Which toxin causes the renal injury?
2. In enteric fever caused by Salmonella typhi, the best method to isolate the organism in the FIRST week of illness is:
3. Enterohemorrhagic E. coli O157:H7 (EHEC) causes hemolytic uremic syndrome (HUS) via Shiga toxin. The molecular mechanism by which Shiga toxin (Stx) kills renal endothelial cells is:
4. Vibrio cholerae O1 El Tor biotype produces cholera toxin (CT) which activates Gs-adenylyl cyclase. However, the specific virulence mechanism enabling V. cholerae to colonize the intestine and avoid peristaltic clearance requires an additional surface structure. This is:
5. Extended-spectrum beta-lactamase (ESBL)-producing Klebsiella pneumoniae is isolated from a patient with a healthcare-associated UTI. The ESBL enzymes are most accurately characterized as:
6. Shigella dysenteriae type 1 is uniquely more virulent among Shigella species primarily because it:
7. ETEC (Enterotoxigenic E. coli) causes watery diarrhea via heat-labile toxin (LT). LT is structurally homologous to cholera toxin. The specific G-protein that LT ADP-ribosylates to cause persistent adenylate cyclase activation is:
8. Salmonella typhi survives within macrophages after phagocytosis. The virulence gene regulatory system that upregulates genes needed for intracellular survival within phagolysosomes is:
9. Vibrio cholerae O139 can cause epidemic cholera whereas most other O serogroups cannot. The unique feature of V. cholerae O139 that allows this is:
10. In EHEC O157:H7 infection, hemolytic uremic syndrome (HUS) develops due to Shiga toxin binding to endothelial Gb3 receptors. The population of renal endothelial cells that expresses the HIGHEST density of Gb3 receptors explaining why kidneys are preferentially affected is:
11. Klebsiella pneumoniae type 1 (classical) pneumonia shows a specific radiographic pattern of 'bulging fissure sign'. The organism-specific virulence factor responsible for the characteristic mucoid and destructive nature of this pneumonia is:
12. Carbapenem-resistant Klebsiella pneumoniae (CRKP) is a critical threat in ICU settings. The most common mechanism of carbapenem resistance in K. pneumoniae in India is production of which enzyme?
13. A 5-year-old child develops bloody diarrhoea followed by haematuria, anaemia (haematocrit 22%), and platelet count of 45,000/µL on day 7 of illness. Stool culture grows lactose-non-fermenting, sorbitol-negative E. coli O157:H7. The toxin responsible for HUS pathogenesis primarily targets which cell type in the kidney?
14. Salmonella Typhi evades killing after phagocytosis by macrophages using which specific intracellular survival strategy?
15. El Tor biotype of Vibrio cholerae O1 differs from the Classical biotype in several respects. Which of the following is the most clinically relevant virulence factor carried by the El Tor strain?
16. Enterohemorrhagic E. coli (EHEC O157:H7) produces Shiga toxin (Stx). After intestinal infection, which mechanism leads to hemolytic uremic syndrome (HUS)?
17. In Salmonella typhi pathogenesis, bacteria survive within macrophages after phagocytosis. Which specific virulence mechanism allows S. typhi to resist phagolysosomal killing in macrophages?
18. Klebsiella pneumoniae producing Extended-Spectrum Beta-Lactamases (ESBLs) is identified in a urine culture. Which phenotypic test confirms ESBL production, and which antibiotic class remains reliably active against most ESBL producers?
19. Vibrio cholerae O1 El Tor biotype produces cholera toxin. Which intracellular signaling mechanism produces the profuse watery diarrhea?
20. Widal test is used in the serological diagnosis of enteric fever. Which antibody titers are considered diagnostically significant in a patient from a non-endemic area without prior typhoid vaccination?
21. A 3-year-old child develops haemolytic uraemic syndrome (HUS) 5 days after a bloody diarrhoeal illness. The causative E. coli produces a toxin that acts by which mechanism to cause endothelial damage?
22. The Widal test in enteric fever measures agglutinating antibodies against Salmonella typhi O and H antigens. A rising titre of O antibody is considered more diagnostically significant than rising H antibody because:
23. A carbapenem-resistant Klebsiella pneumoniae isolate produces an enzyme that is not inhibited by EDTA (a chelator of Zn2+) but is inhibited by clavulanic acid-based inhibitors. This resistance phenotype is most consistent with:
24. Vibrio cholerae cholera toxin (CT) causes severe watery diarrhoea by acting on small intestinal epithelial cells. Its A1 subunit irreversibly ADP-ribosylates which target, and what is the downstream consequence?
25. A neonate develops haemorrhagic enterocolitis and haemolytic uraemic syndrome (HUS) after consuming formula contaminated with E. coli O157:H7. The Shiga toxin (Stx) responsible for HUS acts by which mechanism?
26. Extended-spectrum beta-lactamase (ESBL) producing Klebsiella pneumoniae is isolated from a patient with ICU-acquired pneumonia. ESBLs are predominantly encoded on transferable plasmids. Which antibiotic class is currently the drug of choice for systemic infections caused by ESBL-producing Enterobacterales?
27. Vibrio cholerae El Tor biotype caused the 7th pandemic of cholera. Compared to the classical biotype, El Tor biotype has which property that contributes to its pandemic fitness?
28. A patient from Kerala returns with a 4-day history of bloody diarrhoea, tenesmus, and fever. Stool microscopy shows RBCs, pus cells but no parasites. Stool culture on deoxycholate citrate agar (DCA) grows non-motile, lactose non-fermenting, H2S-negative colonies. Biochemical testing reveals no gas from glucose and no H2S production. The organism is:
29. A child in Bihar develops profuse watery rice-water stools with no fecal leucocytes. Vibrio cholerae O1 El Tor biotype is isolated. Which toxin mechanism accounts for the profuse secretory diarrhoea?
30. A 6-year-old child in Kolkata develops sudden onset bloody diarrhoea followed by oliguria, haematuria and anaemia on day 7. Peripheral smear shows schistocytes. Stool culture reveals E. coli O157:H7. What is the pathogenesis linking the diarrhoea to the renal failure?
31. A patient with known sickle cell disease presents with high fever and bacteraemia. Blood culture grows a gram-negative rod that is non-lactose fermenting, H2S positive on TSI agar, and agglutinates with Salmonella group D antiserum. Which specific Salmonella pathogenicity island (SPI) encodes the type III secretion system responsible for invasion of intestinal epithelial cells?
32. A 45-year-old man with liver cirrhosis develops explosive watery diarrhoea followed by hypotension 12 hours after eating raw oysters at a coastal restaurant. Blood cultures eventually grow a curved gram-negative rod that is halophilic, oxidase-positive and grows on TCBS agar as a yellow colony. What organism is responsible and what explains the particularly high mortality in cirrhotic patients?
33. Campylobacter jejuni infection is the most common bacterial cause of diarrhoea in developed countries. Which post-infectious complication occurring 2–4 weeks after gastroenteritis is associated with Campylobacter, and what is the immunological mechanism?
34. A 50-year-old man with peptic ulcer disease undergoes endoscopic biopsy. The CLO (Campylobacter-Like Organism) test turns pink/magenta within 30 minutes. Which enzyme of Helicobacter pylori is responsible for this colour change, and why does H. pylori produce it?
35. A 55-year-old diabetic patient presents with a rapidly spreading, crepitant swelling of the perineum with blackish necrotic skin. CT shows gas in the fascial planes. Blood culture grows Klebsiella pneumoniae. Which unique finding on laboratory testing distinguishes hypervirulent Klebsiella pneumoniae (hvKP) from classical KP?
36. In the Widal test for enteric fever, which titre on a single serum sample is generally considered significant in an unimmunised patient from an endemic area?
37. Shiga toxin-producing E. coli (STEC/EHEC O157:H7) characteristically does NOT ferment which sugar, distinguishing it from most other E. coli on a selective medium?
38. Vibrio cholerae O139 ('Bengal') strain was first identified in 1992. In contrast to O1 biotypes, O139 is characterised by:
39. A 5-year-old child develops bloody diarrhea followed by oliguric renal failure and thrombocytopenia (platelet 35,000/μL). Peripheral smear shows fragmented RBCs (schistocytes). Stool culture reveals a sorbitol-negative E. coli colony on SMAC (Sorbitol MacConkey agar). The toxin responsible for the renal damage primarily acts by:
40. In Widal test interpretation for enteric fever, a single convalescent titer of anti-O ≥1:160 is significant. Which of the following statements about Widal test limitations is CORRECT?
41. Vibrio parahaemolyticus causes gastroenteritis associated with consumption of seafood. On TCBS (thiosulfate-citrate-bile-salts-sucrose) agar, it forms colonies that are:
42. A 45-year-old diabetic presents with epigastric pain, early satiety, and a positive urea breath test (UBT). 13C-UBT is used for Helicobacter pylori detection. The principle of UBT is based on:
43. A 2-year-old develops HUS (hemolytic uremic syndrome) following bloody diarrhea. Stool culture on sorbitol–MacConkey agar grows colorless colonies (sorbitol non-fermenter). The pathogenic mechanism is:
44. Widal test in a patient at day 10 of enteric fever shows O agglutinins 1:320 and H agglutinins 1:160. A single-sample interpretation in an endemic area should be:
45. A patient returns from Bangladesh with profuse rice-water diarrhea and rapid dehydration. Stool dark-field microscopy shows comma-shaped organisms with darting motility. The El Tor biotype differs from the classical biotype by:
46. A patient with hospital-acquired pneumonia grows Klebsiella pneumoniae on culture. Disk diffusion shows reduced zone to cefotaxime and ceftazidime; adding clavulanic acid to ceftazidime disk restores the zone (zone enhancement ≥5 mm). MIC of meropenem is 0.25 μg/mL. The most appropriate antibiotic is:
47. A urine culture from a 28-year-old woman with recurrent UTI grows E. coli: >10⁵ CFU/mL. Disc diffusion shows resistance to ampicillin, co-trimoxazole, ciprofloxacin, and cefotaxime. Double disc synergy test (DDST) is positive. The carbapenem sensitivity disc shows full sensitivity. What is the most likely resistance mechanism and appropriate empirical treatment?
48. A stool culture from a patient with bloody diarrhoea grows small, non-lactose-fermenting, H₂S-negative colonies on MacConkey and Salmonella-Shigella (SS) agar. Triple sugar iron (TSI) agar shows alkaline slant/acid butt with NO gas and NO H₂S. Motility test is negative. Serotyping reveals Group D antigen. Which organism does this most likely represent?
49. Vibrio cholerae O1 El Tor biotype is isolated from a cholera outbreak. Which set of laboratory tests best confirms this biotype designation compared to the Classical biotype?
50. A patient with typhoid fever has a Widal test done on day 10 of illness showing O antigen titre of 1:160 and H antigen titre of 1:320. No paired serum is available. What is the correct interpretation?
51. A 3-year-old child presents with bloody diarrhoea, pallor, and oliguria 5 days after eating contaminated beef. Peripheral blood smear shows fragmented red cells (schistocytes). Serum creatinine is 4 mg/dL. Stool culture grows E. coli O157:H7. Which virulence factor is primarily responsible for the haemolytic uraemic syndrome (HUS)?
52. A 20-year-old man presents with acute watery diarrhoea producing 'rice-water' stools during a cholera outbreak. Stool dark-field microscopy shows rapidly motile vibrios with 'shooting star' motility that are inhibited on addition of O1 antiserum. Culture on TCBS agar grows yellow colonies. What is the mechanism of cholera toxin?
53. A patient with Salmonella Typhi bacteraemia requires susceptibility testing. The isolate shows reduced susceptibility to ciprofloxacin (MIC 0.125 mg/L; breakpoint for susceptibility ≤0.06 mg/L) but full sensitivity to ceftriaxone and azithromycin. This pattern is best described as:
54. In the laboratory diagnosis of enteric fever, blood culture has the highest yield in the first week of illness. Which culture medium and atmosphere provides optimal recovery of Salmonella Typhi from blood?
55. A 3-year-old child develops bloody diarrhoea followed by oliguria, microangiopathic haemolytic anaemia, and thrombocytopenia. Stool culture grows sorbitol-non-fermenting E. coli on sorbitol-MacConkey agar. The pathogenic mechanism involves:
56. Widal test is performed in a patient with suspected enteric fever. The O antigen titre is 1:160 and H antigen titre is 1:80. A titre of ≥1:160 for O antigen is considered significant. The most appropriate interpretation is:
57. Vibrio cholerae O1 El Tor produces cholera toxin. The action of the A1 subunit of cholera toxin that causes profuse watery diarrhoea is:
58. A 3-year-old child develops hemolytic uremic syndrome (HUS) 7 days after a bloody diarrhea illness. The causal organism produces a toxin that inhibits 60S ribosomal protein synthesis. The blood culture is negative; stool culture grows colonies on sorbitol-MacConkey agar that appear COLOURLESS (sorbitol-non-fermenting). The organism is:
59. Salmonella Typhi bacteremia is confirmed from a blood culture in a 22-year-old. Disk diffusion shows susceptibility to ciprofloxacin (zone 30 mm) but resistance to nalidixic acid (zone 10 mm). What does this nalidixic acid resistance indicate about treatment?
60. Vibrio cholerae produces profuse rice-water diarrhea through which primary toxin mechanism?
61. A 2-year-old child presents with bloody diarrhoea and oliguria. CBC shows microangiopathic haemolytic anaemia and thrombocytopenia. Stool culture grows E. coli that ferments sorbitol slowly. Which E. coli pathotype and toxin are responsible?
62. In Widal's test for typhoid fever, which antibody titre against which antigen is considered diagnostically significant in an unvaccinated individual from a non-endemic area?
63. Stool culture from a 5-year-old with acute bloody diarrhoea grows a lactose non-fermenting organism on MacConkey agar forming colourless colonies. It is non-motile, does not produce H2S, and does not decarboxylate lysine. It ferments mannitol. Which Shigella species is most likely?
64. A Vibrio cholerae O1 El Tor isolate is typed by the Haemagglutinin (HA) protease, sensitivity to Mukherjee's phage IV, and polymyxin B susceptibility. The isolate is sensitive to polymyxin B and sensitive to Mukherjee's phage IV. Which biotype does this represent?
65. A child with bloody diarrhoea develops oliguric acute kidney injury with microangiopathic haemolytic anaemia and thrombocytopenia (HUS). Stool culture reveals a sorbitol-negative Gram-negative bacillus on SMAC (sorbitol MacConkey agar). Which virulence factor is directly responsible for glomerular endothelial injury in HUS?
66. Blood cultures from a patient with enteric fever grow Salmonella Typhi. The Widal test shows anti-O titre 1:160, anti-H titre 1:320 (baseline in the community is 1:80). Ciprofloxacin disk zone is 24 mm (CLSI susceptible ≥31 mm), and nalidixic acid disk zone is 10 mm (resistant). What is the clinical interpretation?
67. A 4-year-old child presents with bloody diarrhoea, and 5 days later develops oliguria and elevated creatinine. Peripheral blood smear shows schistocytes and thrombocytopenia. Stool culture grows an organism that ferments sorbitol slowly and is O157:H7 serotype. The toxin responsible for the haemolytic uremic syndrome (HUS) acts by:
68. In typhoid fever, Widal test interpretation requires knowledge of background titers in endemic areas. A single Widal test showing O titer of 1:160 and H titer of 1:80 in an unimmunised patient from a non-endemic area is:
69. The El Tor biotype of Vibrio cholerae O1 causing pandemic cholera differs from the Classical biotype in that El Tor:
70. A 4-year-old child develops bloody diarrhoea followed 5 days later by pallor, oliguria, and a platelet count of 20,000/µL (normal 1.5–4 lakh). Peripheral smear shows schistocytes. Stool culture grows E. coli that is Sorbitol-MacConkey agar (SMAC) negative (non-sorbitol fermenting). What pathogenic mechanism accounts for the renal injury?
71. Widal test is used for serological diagnosis of typhoid fever. A patient in an endemic area shows Widal titres: TO = 1:160, TH = 1:160. In the absence of prior vaccination or previous illness, which interpretation is MOST appropriate?
72. A patient presents with sudden onset severe watery diarrhoea described as 'rice-water stools,' rapid dehydration, and no fever. Stool dark-field microscopy shows 'shooting star motility.' The MOST appropriate electrolyte replacement solution for adults with severe cholera is:
73. Entero-invasive E. coli (EIEC) and Shigella are clinically and microbiologically similar. The key microbiological distinction between them is:
74. In enteric fever (typhoid), the Widal test becomes positive in the second week of illness. The antigen used to detect Vi antibodies is clinically important because: