A 28-year-old tourist from Germany develops watery diarrhoea without blood or fever on the third day of a holiday in Goa. Stool culture grows lactose-fermenting E. coli, and PCR detects the gene for a heat-stable toxin but not the heat-labile one. How does this toxin cause diarrhoea?

It activates guanylate cyclase and raises cyclic GMP

It ADP-ribosylates Gs and raises cyclic AMP

It inactivates 60S ribosomes in the enterocytes

It invades and destroys the colonic epithelium

A 3-year-old boy in a flood relief camp in Assam has high fever, seizures and frequent stools with blood and mucus. Stool grows Shigella dysenteriae type 1, which produces a potent cytotoxin. How does this toxin damage the cells?

It removes an adenine from 28S rRNA, stopping protein synthesis

It forms pores in the membrane, causing cell lysis

It ADP-ribosylates elongation factor 2, stopping protein synthesis

It ADP-ribosylates Gs alpha, raising cyclic AMP

Gram-negative bacteria and Enterobacterales

26 NEET-PG practice questions on Gram-negative bacteria and Enterobacterales, part of Microbiology. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Medium
A 5-year-old boy develops oliguria, pallor and petechiae a week after bloody diarrhoea. The smear shows schistocytes and platelets are 40,000/µL. Stool grows lactose-fermenting colonies on MacConkey agar that stay colourless on sorbitol MacConkey agar. Which organism is most likely?
1. AShigella dysenteriae type 1
2. BEnteroinvasive E. coli
3. CEnterotoxigenic E. coli
4. DE. coli O157:H7 (EHEC)
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Answer: D · E. coli O157:H7 (EHEC)
The child has haemolytic uraemic syndrome after bloody diarrhoea. Shiga toxin from enterohaemorrhagic E. coli damages glomerular endothelium, and the commonest serotype, O157:H7, differs from most E. coli in not fermenting sorbitol, so its colonies are colourless on sorbitol MacConkey agar. Antibiotics are generally avoided in suspected EHEC because they may increase toxin release and the risk of HUS. Management of HUS is supportive, with dialysis when needed.
- A: Shigella dysenteriae type 1 can also cause HUS in India, but it does not ferment lactose.
- B: Enteroinvasive E. coli causes dysentery without Shiga toxin and does not cause HUS.
- C: Enterotoxigenic E. coli causes watery travellers' diarrhoea, not bloody diarrhoea or HUS.
Question 2 · Medium
A 28-year-old tourist from Germany develops watery diarrhoea without blood or fever on the third day of a holiday in Goa. Stool culture grows lactose-fermenting E. coli, and PCR detects the gene for a heat-stable toxin but not the heat-labile one. How does this toxin cause diarrhoea?
1. AIt activates guanylate cyclase and raises cyclic GMP
2. BIt ADP-ribosylates Gs and raises cyclic AMP
3. CIt inactivates 60S ribosomes in the enterocytes
4. DIt invades and destroys the colonic epithelium
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Answer: A · It activates guanylate cyclase and raises cyclic GMP
Watery travellers' diarrhoea with an E. coli carrying a toxin gene is enterotoxigenic E. coli. Its heat-stable toxin (ST) binds guanylate cyclase C on enterocytes, raising cyclic GMP, which drives chloride secretion and reduces sodium absorption. The heat-labile toxin (LT), like cholera toxin, works through Gs and cyclic AMP instead. The illness is usually self-limiting and is managed mainly with oral rehydration.
- B: ADP-ribosylation of Gs is the action of the heat-labile toxin, not the heat-stable one.
- C: Inactivation of 60S ribosomes is the action of Shiga toxins.
- D: Invasion of colonic epithelium is the mechanism of Shigella and enteroinvasive E. coli.
Question 3 · Medium
A 3-year-old boy in a flood relief camp in Assam has high fever, seizures and frequent stools with blood and mucus. Stool grows Shigella dysenteriae type 1, which produces a potent cytotoxin. How does this toxin damage the cells?
1. AIt forms pores in the membrane, causing cell lysis
2. BIt ADP-ribosylates elongation factor 2, stopping protein synthesis
3. CIt ADP-ribosylates Gs alpha, raising cyclic AMP
4. DIt removes an adenine from 28S rRNA, stopping protein synthesis
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Answer: D · It removes an adenine from 28S rRNA, stopping protein synthesis
Shigella dysenteriae type 1 produces Shiga toxin, an AB5 toxin that binds the glycolipid Gb3. Its A subunit is an N-glycosidase that removes an adenine from 28S rRNA of the 60S ribosomal subunit, halting protein synthesis and killing intestinal and glomerular endothelial cells. This explains the severe dysentery and the risk of haemolytic uraemic syndrome with this serotype. Children in camps need ORS, an effective antibiotic and good sanitation.
- A: Pore formation is the action of toxins such as alpha-haemolysin of S. aureus.
- B: ADP-ribosylation of EF-2 is the action of diphtheria toxin and Pseudomonas exotoxin A.
- C: ADP-ribosylation of Gs is the action of cholera toxin.
Question 4 · Easy
A 30-year-old woman with 35% burns in a Delhi burns unit develops fever on day 9. Her dressings have a blue-green colour and a grape-like smell, and the swab grows oxidase-positive, non-lactose-fermenting Gram-negative bacilli. Which pigment produces the blue-green colour?
1. APyoverdin
2. BStaphyloxanthin
3. CPyocyanin
4. DProdigiosin
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Answer: C · Pyocyanin
Blue-green discharge with a sweet grape-like odour from a burn wound is typical of Pseudomonas aeruginosa, an oxidase-positive non-fermenter. Pyocyanin is the blue, chloroform-soluble phenazine pigment that, with yellow-green pyoverdin, gives the blue-green colour; it also generates reactive oxygen species that damage tissue. Burn wound sepsis with Pseudomonas is treated with an antipseudomonal agent chosen by susceptibility, along with wound care.
- A: Pyoverdin is yellow-green and fluorescent, not blue.
- B: Staphyloxanthin is the golden pigment of S. aureus.
- D: Prodigiosin is the red pigment of Serratia marcescens.
Question 5 · Easy
CSF from a 9-month-old girl in Siliguri, whose family missed her vaccinations, shows tiny pleomorphic Gram-negative coccobacilli. The laboratory streaks Staphylococcus aureus across a sheep blood agar plate inoculated with the CSF; next day, minute colonies appear only beside the staphylococcal line. What does S. aureus supply to these colonies?
1. AHaemin (factor X)
2. BCarbon dioxide
3. CCysteine and iron
4. DNAD (factor V)
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Answer: D · NAD (factor V)
Satellitism of small Gram-negative coccobacilli around S. aureus identifies Haemophilus influenzae, the likely cause of meningitis in this unimmunised infant. H. influenzae needs both factor X (haemin) and factor V (NAD). Blood agar contains factor X, but the NAD within the red cells is not available; S. aureus releases NAD through haemolysis, so colonies grow only near it. Chocolate agar, in which heating releases both factors, supports growth everywhere on the plate.
- A: Haemin is already present in blood agar and is not what S. aureus supplies.
- B: Carbon dioxide enrichment helps growth but does not explain satellitism.
- C: Cysteine and iron are needed by Legionella, not Haemophilus.
Question 6 · Medium
A 20-year-old student is admitted in a Pune hospital with confirmed meningococcal meningitis. The district surveillance officer must decide who receives chemoprophylaxis. 1. His roommate in the hostel. 2. A resident who intubated him in casualty without wearing a mask. 3. Students who sat in the same lecture hall last week. 4. A nurse who started his IV line and gave drugs. Which people need chemoprophylaxis?
1. A1 and 3 only
2. B1, 2 and 4 only
3. C1 and 2 only
4. D1, 2, 3 and 4
Answer and explanation in LearnFree account
Question 7 · Easy
On day 3 of life, a baby born by vaginal delivery at home in rural Rajasthan has marked swelling of both eyelids and a thick, copious yellow discharge from both eyes. The mother had vaginal discharge in late pregnancy that was not treated. What is the most likely cause?
1. AHerpes simplex virus
2. BNeisseria gonorrhoeae
3. CChlamydia trachomatis
4. DSilver nitrate drops
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Question 8 · Easy
A 35-year-old man from a riverside village in Odisha has profuse painless rice-water stools and severe dehydration. The PHC can send stool to the district laboratory only the next morning. Which transport medium is most suitable for the specimen?
1. ARobertson cooked meat medium
2. BLoeffler serum slope
3. CSelenite F broth
4. DVenkatraman-Ramakrishnan medium
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Question 9 · Medium
During monsoon, a 40-year-old man in a Kolkata slum passes about a litre of grey rice-water stool every hour, without fever or abdominal pain. Stool culture grows Vibrio cholerae O1. What is the action of the toxin responsible?
1. ARemoval of adenine from 28S rRNA, which stops protein synthesis
2. BActivation of guanylate cyclase C, which raises cyclic GMP
3. CADP-ribosylation of Gi alpha, which blocks inhibition of adenylate cyclase
4. DADP-ribosylation of Gs alpha, which keeps adenylate cyclase active
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Question 10 · Medium
A 35-year-old woman receiving chemotherapy for acute myeloid leukaemia in a Mumbai hospital has a temperature of 39°C and an absolute neutrophil count of 200/µL. On her thigh is a painless, round, black necrotic ulcer with a red halo. BP is 110/70 mmHg. Blood cultures are taken. Which empirical antibiotic is most appropriate?
1. ATigecycline
2. BPiperacillin-tazobactam
3. CCeftriaxone
4. DErtapenem
Answer and explanation in LearnFree account
Question 11 · Medium
Eleven people develop abdominal cramps and watery diarrhoea, some with a little blood, about 16 hours after eating undercooked prawns at a seaside restaurant in Mangaluru. Stool culture on TCBS agar grows green colonies of a curved Gram-negative bacillus that grows in 8% sodium chloride but not without salt, and shows beta-haemolysis on Wagatsuma agar. Which organism is this?
1. AAeromonas hydrophila
2. BVibrio cholerae O1
3. CCampylobacter jejuni
4. DVibrio parahaemolyticus
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Question 12 · Medium
An 18-month-old unimmunised boy from a family that refused vaccines is admitted in Moradabad with meningitis due to Haemophilus influenzae type b. The paediatrician counsels the parents about the vaccine for his younger sibling. Which component of the organism does this vaccine contain?
1. AIgA1 protease as a toxoid
2. BPolyribosyl ribitol phosphate capsule
3. CLipooligosaccharide of the outer membrane
4. DInactivated whole bacterial cells
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Question 13 · Hard
A 17-year-old girl is brought to casualty in Jammu with fever and vomiting for 12 hours. She is drowsy, with neck stiffness and a non-blanching purpuric rash on the legs. Pulse 136/min, BP 80/50 mmHg, capillary refill 5 seconds, platelets 60,000/µL and INR 2.0. What is the next best step?
1. ACT head first, then antibiotics if no mass is seen
2. BLumbar puncture before starting any antibiotic
3. CAwait Gram stain of a rash aspirate before treating
4. DIV ceftriaxone at once, after taking blood cultures
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Question 14 · Hard
A 63-year-old diabetic woman in a Ludhiana hospital has urosepsis with BP 88/56 mmHg. Blood and urine grow E. coli. Disc diffusion: ceftazidime 13 mm, ceftazidime-clavulanate 22 mm; cefotaxime 15 mm, cefotaxime-clavulanate 24 mm; cefoxitin susceptible; meropenem susceptible. By CLSI, a rise of 5 mm or more with clavulanate is significant. Which antibiotic is most appropriate?
1. ACeftriaxone
2. BAmoxicillin-clavulanate
3. CMeropenem
4. DPiperacillin-tazobactam
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Question 15 · Easy
A 26-year-old truck driver attends the STI clinic of a district hospital with a profuse purulent urethral discharge and burning micturition for three days. Gram stain shows Gram-negative intracellular diplococci in pus cells. Which drug is preferred for the gonococcal infection?
1. ACiprofloxacin
2. BCeftriaxone
3. CCotrimoxazole
4. DBenzathine penicillin
Answer and explanation in LearnFree account
Question 16 · Medium
A 25-year-old man attends the STI clinic in Visakhapatnam with two painful ulcers on the penis for a week. They have ragged, undermined edges and a dirty, easily bleeding base. A tender inguinal swelling is fluctuant. Gram stain of the ulcer base shows small Gram-negative bacilli in parallel chains like a school of fish. What is the most likely organism?
1. AChlamydia trachomatis L2
2. BTreponema pallidum
3. CKlebsiella granulomatis
4. DHaemophilus ducreyi
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Question 17 · Medium
A 2-year-old girl in Vellore has had watery diarrhoea with mucus for 18 days and is losing weight. Stool culture grows E. coli. In the reference laboratory, the isolate sticks to HEp-2 cells and to the glass between them in a stacked-brick pattern. Which pathotype is this?
1. AEnteropathogenic E. coli
2. BEnteroinvasive E. coli
3. CEnterohaemorrhagic E. coli
4. DEnteroaggregative E. coli
Answer and explanation in LearnFree account
Question 18 · Easy
In a district public health laboratory in Darbhanga, a stool isolate from a toddler with dysentery forms pale colonies on MacConkey agar. On TSI it gives an alkaline slant over an acid butt, with no gas and no blackening; it is non-motile in a hanging drop and urease negative. Which organism is this?
1. AProteus mirabilis
2. BShigella
3. CSalmonella Typhi
4. DEscherichia coli
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Question 19 · Medium
Five infants under one year in the paediatric ward of a Ranchi hospital develop watery diarrhoea over one week. Stool from each grows E. coli that is negative for Shiga toxin and enterotoxin genes but carries the eae gene encoding intimin. Biopsy would show bacteria tightly attached to enterocytes on raised pedestals. How do these bacteria cause diarrhoea?
1. ASecreting a toxin that raises cyclic AMP
2. BInvading and spreading between colonocytes
3. CStacking on mucosa as a thick biofilm
4. DAttaching and effacing the microvilli
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Question 20 · Hard
A 60-year-old man in a Hyderabad ICU develops ventilator-associated pneumonia. Tracheal aspirate grows a non-lactose-fermenting Gram-negative bacillus with a fruity smell and a metallic sheen. The resident lists four features: 1. It is oxidase positive and does not ferment glucose. 2. It produces pyoverdin, a fluorescent siderophore. 3. Its exotoxin A ADP-ribosylates elongation factor 2. 4. It is reliably susceptible to ertapenem. Which statements are correct?
1. A1 and 4 only
2. B1, 2 and 3 only
3. C2, 3 and 4 only
4. D1 and 3 only
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Question 21 · Medium
A 50-year-old woman in Coimbatore with recurrent urinary infections has a staghorn calculus in the left kidney. Urine pH is 8.0, and urine culture grows a Gram-negative bacillus that spreads in waves over the whole surface of a blood agar plate. Which enzyme of this organism explains the stone?
1. AUrease
2. BCoagulase
3. CBeta-lactamase
4. DLecithinase
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Question 22 · Medium
A 9-year-old boy with sickle cell anaemia from a tribal district of Chhattisgarh has fever and a painful swollen tibia. Bone aspirate grows motile, non-lactose-fermenting, H2S-producing Gram-negative bacilli that agglutinate with Salmonella polyvalent O antiserum. Which feature of his disease most predisposes him to this infection?
1. AIron overload from repeated transfusion
2. BLow immunoglobulin from protein loss
3. CFunctional asplenia with infarcted bone
4. DNeutropenia from hydroxyurea therapy
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Question 23 · Hard
A 55-year-old man with poorly controlled diabetes in a Bhubaneswar hospital has fever for 10 days and a single 6 cm abscess in the right lobe of the liver. Two days after admission, he complains of pain and blurred vision in the left eye. Aspirated pus grows a lactose-fermenting, non-motile Gram-negative bacillus forming very mucoid colonies; touching a colony with a loop draws out a viscous string 8 mm long. Which organism is most likely?
1. ABurkholderia pseudomallei
2. BEntamoeba histolytica
3. CEscherichia coli
4. DHypervirulent Klebsiella pneumoniae
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Question 24 · Hard
A 22-year-old man from Agra has had fever for three weeks with a coated tongue, a pulse of 84/min at 39.5°C and a palpable spleen. A local practitioner gave him 10 days of cefixime, which he stopped a week ago. Two blood cultures now are sterile, and a Widal test is equivocal. Typhoid is still suspected. Which investigation has the best yield now?
1. ABone marrow culture
2. BStool culture
3. CRepeat Widal test after one week
4. DUrine culture
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Question 25 · Hard
A 67-year-old man in a Delhi ICU has ventilator-associated pneumonia with Klebsiella pneumoniae resistant to all beta-lactams including meropenem. The isolate carries blaNDM-1, and the carbapenemase activity is inhibited by EDTA. It also produces an extended-spectrum beta-lactamase. Which regimen is most appropriate?
1. AHigh-dose extended-infusion meropenem
2. BCeftazidime-avibactam with aztreonam
3. CCeftazidime-avibactam alone
4. DPiperacillin-tazobactam
Answer and explanation in LearnFree account

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Gram-negative bacteria and Enterobacterales

A 5-year-old boy develops oliguria, pallor and petechiae a week after bloody diarrhoea. The smear shows schistocytes and platelets are 40,000/µL. Stool grows lactose-fermenting colonies on MacConkey agar that stay colourless on sorbitol MacConkey agar. Which organism is most likely?

A 28-year-old tourist from Germany develops watery diarrhoea without blood or fever on the third day of a holiday in Goa. Stool culture grows lactose-fermenting E. coli, and PCR detects the gene for a heat-stable toxin but not the heat-labile one. How does this toxin cause diarrhoea?

A 3-year-old boy in a flood relief camp in Assam has high fever, seizures and frequent stools with blood and mucus. Stool grows Shigella dysenteriae type 1, which produces a potent cytotoxin. How does this toxin damage the cells?

A 30-year-old woman with 35% burns in a Delhi burns unit develops fever on day 9. Her dressings have a blue-green colour and a grape-like smell, and the swab grows oxidase-positive, non-lactose-fermenting Gram-negative bacilli. Which pigment produces the blue-green colour?

On day 3 of life, a baby born by vaginal delivery at home in rural Rajasthan has marked swelling of both eyelids and a thick, copious yellow discharge from both eyes. The mother had vaginal discharge in late pregnancy that was not treated. What is the most likely cause?

A 35-year-old man from a riverside village in Odisha has profuse painless rice-water stools and severe dehydration. The PHC can send stool to the district laboratory only the next morning. Which transport medium is most suitable for the specimen?

During monsoon, a 40-year-old man in a Kolkata slum passes about a litre of grey rice-water stool every hour, without fever or abdominal pain. Stool culture grows Vibrio cholerae O1. What is the action of the toxin responsible?

An 18-month-old unimmunised boy from a family that refused vaccines is admitted in Moradabad with meningitis due to Haemophilus influenzae type b. The paediatrician counsels the parents about the vaccine for his younger sibling. Which component of the organism does this vaccine contain?

A 17-year-old girl is brought to casualty in Jammu with fever and vomiting for 12 hours. She is drowsy, with neck stiffness and a non-blanching purpuric rash on the legs. Pulse 136/min, BP 80/50 mmHg, capillary refill 5 seconds, platelets 60,000/µL and INR 2.0. What is the next best step?

A 26-year-old truck driver attends the STI clinic of a district hospital with a profuse purulent urethral discharge and burning micturition for three days. Gram stain shows Gram-negative intracellular diplococci in pus cells. Which drug is preferred for the gonococcal infection?

A 2-year-old girl in Vellore has had watery diarrhoea with mucus for 18 days and is losing weight. Stool culture grows E. coli. In the reference laboratory, the isolate sticks to HEp-2 cells and to the glass between them in a stacked-brick pattern. Which pathotype is this?

A 50-year-old woman in Coimbatore with recurrent urinary infections has a staghorn calculus in the left kidney. Urine pH is 8.0, and urine culture grows a Gram-negative bacillus that spreads in waves over the whole surface of a blood agar plate. Which enzyme of this organism explains the stone?