A 40-year-old farmer from Bihar stepped on a rusty nail two weeks ago. He now has difficulty opening his mouth, a fixed grin, and painful spasms of the back and abdominal muscles that are set off by noise. He is fully conscious. How does the toxin produce these spasms?

It blocks glycine and GABA release from inhibitory interneurons

It stimulates acetylcholine release at the motor end plate

It blocks acetylcholine release at the neuromuscular junction

It destroys anterior horn cells of the spinal cord

A 5-year-old unimmunised girl from a village in Assam has fever and a dirty grey membrane over the tonsils and soft palate. The medical officer sends a throat swab to the district laboratory. Which medium gives the earliest growth of the suspected organism?

Thiosulfate citrate bile salt sucrose agar

Gram-positive cocci and bacilli

26 NEET-PG practice questions on Gram-positive cocci and bacilli, 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 · Easy
A 6-year-old unimmunised boy from a village in Uttar Pradesh has fever, a swollen 'bull neck' and a greyish tonsillar membrane that bleeds when scraped. Consider four statements about the causative organism: 1. Its toxin ADP-ribosylates elongation factor 2. 2. The toxin gene is carried by a lysogenic beta-phage. 3. Albert stain shows metachromatic granules in the bacilli. 4. Toxigenicity of an isolate can be shown by the Elek gel precipitation test. Which statements are correct?
1. A1, 3 and 4 only
2. B1, 2, 3 and 4
3. C1 and 3 only
4. D2, 3 and 4 only
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Answer: B · 1, 2, 3 and 4
All four statements are correct for Corynebacterium diphtheriae. The toxin's B fragment binds the cell and the A fragment ADP-ribosylates elongation factor 2, stopping protein synthesis in the throat, heart and nerves. Only strains lysogenised by a beta-phage carrying the tox gene make toxin, so an isolate's toxigenicity is shown by the Elek test (or PCR for the tox gene), and Albert stain shows the metachromatic (volutin) granules in a smear. Diphtheria antitoxin is given on clinical suspicion, without waiting for culture.
- A: Omits statement 2; the tox gene is indeed carried by a phage.
- C: Omits the phage-borne toxin gene and the Elek test, both correct.
- D: Omits statement 1, the correct mechanism of the toxin.
Question 2 · Medium
Three members of a family in a Manipur village are admitted to the district hospital a day after eating fermented fish from a sealed jar. They have double vision, drooping eyelids, slurred speech and difficulty swallowing, with weakness that spreads down to the arms. They are afebrile and alert. The physician makes four statements: 1. The toxin blocks acetylcholine release by cleaving SNARE proteins. 2. Sensory loss in the limbs is expected. 3. The pupils may be dilated and poorly reactive. 4. Aminoglycosides may worsen the weakness. Which statements are correct?
1. A1, 3 and 4 only
2. B2 and 3 only
3. C1, 2 and 4 only
4. D1 and 4 only
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Answer: A · 1, 3 and 4 only
Statements 1, 3 and 4 are correct. Foodborne botulism from preformed toxin causes a descending, symmetrical flaccid paralysis starting with the cranial nerves, in an alert and afebrile patient. The toxin is a zinc protease that cleaves SNARE proteins needed for acetylcholine release at the neuromuscular junction and at autonomic cholinergic endings, which is why dilated pupils, dry mouth and constipation occur; aminoglycosides further impair neuromuscular transmission. Statement 2 is wrong because sensation is preserved; the key treatment is early antitoxin, with ventilatory support as needed.
- B: Includes sensory loss and omits statements 1 and 4.
- C: Includes sensory loss, which is absent in botulism, and omits the pupils.
- D: Omits statement 3; dilated, poorly reactive pupils do occur.
Question 3 · Medium
A neonate born at 38 weeks in a district hospital develops grunting, poor feeding and temperature instability 12 hours after birth. Membranes had ruptured 22 hours before delivery. Blood culture grows Gram-positive cocci in chains with a narrow zone of beta-haemolysis, resistant to bacitracin, positive on hippurate hydrolysis, and showing an arrowhead of enhanced haemolysis next to a Staphylococcus aureus streak. Which organism is this?
1. AStreptococcus dysgalactiae
2. BStreptococcus gallolyticus
3. CStreptococcus pyogenes
4. DStreptococcus agalactiae
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Answer: D · Streptococcus agalactiae
Early-onset neonatal sepsis after prolonged rupture of membranes with a beta-haemolytic, bacitracin-resistant, hippurate-positive streptococcus is group B streptococcus. The arrowhead of haemolysis is the CAMP test: the CAMP factor of S. agalactiae enhances the beta-lysin of S. aureus. The organism colonises the maternal genital tract and is acquired during delivery. Intrapartum penicillin or ampicillin to colonised or high-risk mothers reduces early-onset disease.
- A: S. dysgalactiae (groups C and G) is bacitracin resistant but CAMP and hippurate negative.
- B: S. gallolyticus (group D) is usually non-haemolytic and bile esculin positive.
- C: S. pyogenes is bacitracin sensitive and CAMP negative.
Question 4 · Medium
A 40-year-old farmer from Bihar stepped on a rusty nail two weeks ago. He now has difficulty opening his mouth, a fixed grin, and painful spasms of the back and abdominal muscles that are set off by noise. He is fully conscious. How does the toxin produce these spasms?
1. AIt blocks glycine and GABA release from inhibitory interneurons
2. BIt stimulates acetylcholine release at the motor end plate
3. CIt blocks acetylcholine release at the neuromuscular junction
4. DIt destroys anterior horn cells of the spinal cord
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Answer: A · It blocks glycine and GABA release from inhibitory interneurons
This is generalised tetanus. Tetanospasmin enters motor nerve endings, travels by retrograde axonal transport to the spinal cord and brainstem, and cleaves synaptobrevin in inhibitory interneurons (including Renshaw cells). Glycine and GABA are no longer released, so motor neurons fire unchecked, causing rigidity and reflex spasms. Treatment includes human tetanus immunoglobulin to neutralise unbound toxin, wound care, metronidazole, and benzodiazepines in a quiet ICU.
- B: No clostridial toxin stimulates acetylcholine release; the spasms come from lost inhibition.
- C: Blocking acetylcholine release at the NMJ is botulinum toxin, which causes flaccid paralysis.
- D: Destruction of anterior horn cells is poliomyelitis, causing flaccid weakness.
Question 5 · Easy
A 5-year-old unimmunised girl from a village in Assam has fever and a dirty grey membrane over the tonsils and soft palate. The medical officer sends a throat swab to the district laboratory. Which medium gives the earliest growth of the suspected organism?
1. APotassium tellurite blood agar
2. BThiosulfate citrate bile salt sucrose agar
3. CLoeffler serum slope
4. DThayer-Martin agar
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Answer: C · Loeffler serum slope
A grey pharyngeal membrane in an unimmunised child suggests diphtheria. Corynebacterium diphtheriae grows rapidly on Loeffler serum slope, with colonies visible in 6 to 8 hours, and smears from it show the characteristic metachromatic granules on Albert stain. Tellurite blood agar is selective and helps separate biotypes by colony, but growth takes 1 to 2 days. Antitoxin is given on clinical suspicion without waiting for the culture.
- A: Tellurite agar is selective, but its black colonies take 1 to 2 days to appear.
- B: TCBS agar is selective for Vibrio species.
- D: Thayer-Martin agar is selective for pathogenic Neisseria.
Question 6 · Easy
A 5-year-old boy from a crowded settlement in Kolkata had honey-coloured crusted sores around his nose and mouth three weeks ago, treated with a home remedy. He now has puffy eyes, smoky urine and a BP of 128/86 mmHg. Which complication of this skin infection is he showing?
1. APost-streptococcal glomerulonephritis
2. BAcute rheumatic fever
3. CHaemolytic uraemic syndrome
4. DStaphylococcal scalded skin syndrome
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Question 7 · Hard
A 45-year-old wool sorter in Panipat is admitted with fever, chest discomfort and breathlessness after a flu-like illness. Chest X-ray shows a widened mediastinum with pleural effusions. Blood culture grows large Gram-positive bacilli. The microbiologist lists four features of the organism: 1. Its capsule is made of poly-D-glutamic acid. 2. Colonies on agar show a Medusa head appearance. 3. It is actively motile in a hanging drop. 4. It forms a string of pearls on agar containing a low concentration of penicillin. Which features are correct?
1. A2, 3 and 4 only
2. B1, 2 and 4 only
3. C1 and 3 only
4. D2 and 4 only
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Question 8 · Medium
A 45-year-old diabetic man in a Jaipur hospital has a rapidly spreading, exquisitely painful swelling of the thigh with dusky skin, bullae and fever, two days after a minor cut. At surgery the fascia is grey and necrotic, and Gram stain shows Gram-positive cocci in chains. Debridement and IV penicillin are started. Why is clindamycin added?
1. AIt is bactericidal where penicillin is only bacteriostatic
2. BIt neutralises circulating toxin already in the blood
3. CIt widens cover to Gram-negative bacilli in the wound
4. DIt suppresses toxin synthesis regardless of growth phase
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Question 9 · Hard
In a NICU in Bhopal, four neonates develop MRSA bloodstream infection in three weeks, with the same antibiogram. The infection control committee considers four measures: 1. Screen nasal swabs of NICU staff for MRSA carriage. 2. Apply contact precautions and cohort the infected babies. 3. Give mupirocin nasal ointment to carriers found on screening. 4. Start prophylactic IV vancomycin for every baby admitted to the NICU. Which measures are appropriate?
1. A2 and 3 only
2. B1, 2 and 3 only
3. C1, 2 and 4 only
4. D1 and 4 only
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Question 10 · Medium
A 70-year-old man is treated in a Nagpur hospital ward with IV ceftriaxone for a urinary tract infection after catheterisation, but his fever persists. Urine culture grows Gram-positive cocci in pairs and short chains that grow in 6.5% sodium chloride, hydrolyse esculin in the presence of bile and are PYR positive. Why has the treatment failed?
1. AThe organism produces a plasmid extended-spectrum beta-lactamase
2. BThe organism is intrinsically resistant to cephalosporins
3. CCeftriaxone does not reach the urine in adequate levels
4. DCeftriaxone needs an aminoglycoside to act on Gram-positive cocci
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Question 11 · Medium
A 30-year-old man is brought to a trauma centre in Kanpur with a crushed leg from a road accident two days ago. The leg is swollen and bronze with blebs, there is crepitus, and the discharge smells foul. Gram stain shows large, boxcar-shaped Gram-positive bacilli without spores and very few pus cells. On egg yolk agar, opalescence around the colonies is prevented on the half of the plate spread with antitoxin. Which toxin does this reaction demonstrate?
1. AAlpha toxin, a lecithinase
2. BEpsilon toxin, a permease
3. CEnterotoxin, a pore-forming protein
4. DTheta toxin, a haemolysin
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Question 12 · Hard
A 74-year-old woman in a Kochi medical ward develops profuse watery diarrhoea on day 8 of IV ceftriaxone for pneumonia. She is on pantoprazole. Temperature 38.4°C, total leucocyte count 19,500/µL and creatinine 1.8 mg/dL (baseline 0.9). Stool is positive for glutamate dehydrogenase antigen and for toxin A/B by EIA. She is alert, BP is 116/70 mmHg and there is no ileus. Ceftriaxone is stopped. What is the most appropriate treatment?
1. AOral metronidazole
2. BIV vancomycin
3. COral vancomycin
4. DOral loperamide
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Question 13 · Medium
A 3-year-old girl is brought to a paediatric OPD in Indore with fever and widespread tender redness that has peeled in large sheets, leaving moist raw skin. Gentle rubbing of normal-looking skin makes it slide off. Her lips and mouth are not involved. Culture from a nasal swab grows Staphylococcus aureus; blister fluid is sterile. What does the responsible toxin act on?
1. ADesmoglein 3 in the suprabasal layer
2. BKeratinocytes through T-cell cytotoxicity
3. CType VII collagen in anchoring fibrils
4. DDesmoglein 1 in the granular layer
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Question 14 · Medium
A 32-year-old woman is brought to casualty in Pune with a temperature of 39.8°C, vomiting, watery diarrhoea and a diffuse sunburn-like rash two days after septoplasty with nasal packing. BP is 78/46 mmHg, platelets 82,000/µL and creatinine 2.1 mg/dL. The nasal pack grows Staphylococcus aureus; blood cultures are sterile. How does the toxin involved activate T cells?
1. ABridges MHC class II and the T-cell receptor beta chain outside the peptide groove
2. BADP-ribosylates the Gs protein inside T cells and raises cyclic AMP
3. CIs processed by macrophages and presented as a peptide in the MHC II groove
4. DBinds CD28 on T cells directly and replaces the costimulatory signal
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Question 15 · Easy
Pus from a breast abscess in a lactating mother in a Patna hospital grows golden-yellow colonies of catalase-positive Gram-positive cocci in clusters. Which test best separates Staphylococcus aureus from coagulase-negative staphylococci?
1. AOxidase test
2. BCoagulase test
3. CCatalase test
4. DBile solubility test
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Question 16 · Medium
A 62-year-old man is admitted in Bengaluru with fever, neck stiffness and confusion. CSF shows 2,400 cells/µL, mostly neutrophils, glucose 18 mg/dL and many Gram-positive lanceolate diplococci. The isolate is reported resistant to penicillin by MIC, but no beta-lactamase is detected. What is the mechanism of resistance?
1. AAltered penicillin-binding proteins
2. BProduction of an extended-spectrum beta-lactamase
3. CActive efflux of the drug from the cell
4. DLoss of outer membrane porin channels
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Question 17 · Hard
A 26-year-old construction worker in Ahmedabad comes to casualty with a deep laceration of the leg contaminated with soil and manure. His immunisation card shows a complete primary series and boosters in childhood, the last tetanus-containing dose given 7 years ago. The wound is cleaned and debrided. What tetanus prophylaxis does he need?
1. ANo tetanus prophylaxis
2. BTetanus immunoglobulin alone
3. CTd vaccine and tetanus immunoglobulin
4. DTd vaccine alone
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Question 18 · Easy
Two hours after a dinner of leftover fried rice from a roadside eatery in Mumbai, four college students develop nausea and repeated vomiting without fever. The rice had been boiled in the morning and kept at room temperature before frying. Which organism is most likely responsible?
1. ASalmonella Enteritidis
2. BBacillus cereus
3. CVibrio parahaemolyticus
4. DClostridium perfringens
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Question 19 · Hard
A 48-year-old woman on chemotherapy for leukaemia in a Delhi ICU develops a bloodstream infection. Blood grows Enterococcus faecium resistant to ampicillin, vancomycin and teicoplanin, but susceptible to linezolid. The microbiologist reports that the resistance gene is on a transposon carried by a plasmid. How does this gene make the organism resistant to vancomycin?
1. AIt produces an enzyme that hydrolyses the vancomycin molecule
2. BIt alters the 50S ribosomal target of vancomycin binding
3. CIt thickens the outer membrane so vancomycin cannot enter
4. DIt changes the peptidoglycan precursor end to D-Ala-D-Lac
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Question 20 · Medium
A 38-year-old man who handles goat hides at a tannery in Kanpur has a painless lesion on the forearm. It began as an itchy papule, became a vesicle and is now a black eschar with marked non-pitting oedema around it. A smear from the lesion shows large Gram-positive bacilli. Which staining reaction demonstrates the capsule of the suspected organism?
1. AZiehl-Neelsen stain with 5% sulphuric acid
2. BMcFadyean reaction with polychrome methylene blue
3. CAlbert stain for metachromatic granules
4. DNagler reaction on egg yolk agar
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Question 21 · Medium
A 21-year-old newly married woman in Thiruvananthapuram has burning micturition and frequency. Urine culture grows more than 100,000 CFU/mL of catalase-positive, coagulase-negative Gram-positive cocci in clusters. The isolate grows up to the edge of a novobiocin disc. Which organism is this?
1. AStaphylococcus epidermidis
2. BStaphylococcus haemolyticus
3. CStaphylococcus saprophyticus
4. DEnterococcus faecalis
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Question 22 · Medium
A 7-year-old boy in a Malappuram ward is confirmed to have diphtheria by culture and toxigenicity testing. His 4-year-old sister, who shares his bed, is well, and her vaccination card shows only one dose of pentavalent vaccine. What should be done for the sister?
1. ADiphtheria antitoxin alone, with no swab
2. BA single booster of vaccine with no antibiotic
3. CThroat swab, erythromycin and catch-up vaccination
4. DObservation only, as she has no symptoms
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Question 23 · Easy
Within three hours of a wedding lunch in Lucknow, 22 guests develop severe vomiting and abdominal cramps without fever. All ate a milk-based sweet that was prepared the previous evening by a cook with a boil on his thumb, and then reheated before serving. Why did reheating fail to prevent the outbreak?
1. AA preformed toxin that is heat-stable
2. BA capsule that resists heat
3. CBacteria that multiply in the gut
4. DSpores that survive boiling
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Question 24 · Medium
A 64-year-old man who had a kidney transplant two years ago and takes tacrolimus and prednisolone presents in Lucknow with fever, headache and drowsiness. CSF shows 650 cells/µL with 70% neutrophils and glucose 32 mg/dL, and Gram stain shows a few short Gram-positive bacilli. Empirical IV ceftriaxone and vancomycin have been started. Which drug must be added?
1. AAmpicillin
2. BDoxycycline
3. CMetronidazole
4. DAmikacin
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Question 25 · Medium
A 66-year-old man from Hyderabad has fever for three weeks. Echocardiography shows a vegetation on the aortic valve, and blood cultures grow Streptococcus gallolyticus (formerly S. bovis biotype I). He is started on IV penicillin. Which further investigation is most important for him?
1. AUpper GI endoscopy
2. BColonoscopy
3. CAbdominal ultrasound
4. DDental radiograph
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Gram-positive cocci and bacilli

A 40-year-old farmer from Bihar stepped on a rusty nail two weeks ago. He now has difficulty opening his mouth, a fixed grin, and painful spasms of the back and abdominal muscles that are set off by noise. He is fully conscious. How does the toxin produce these spasms?

A 5-year-old unimmunised girl from a village in Assam has fever and a dirty grey membrane over the tonsils and soft palate. The medical officer sends a throat swab to the district laboratory. Which medium gives the earliest growth of the suspected organism?

A 5-year-old boy from a crowded settlement in Kolkata had honey-coloured crusted sores around his nose and mouth three weeks ago, treated with a home remedy. He now has puffy eyes, smoky urine and a BP of 128/86 mmHg. Which complication of this skin infection is he showing?

Pus from a breast abscess in a lactating mother in a Patna hospital grows golden-yellow colonies of catalase-positive Gram-positive cocci in clusters. Which test best separates Staphylococcus aureus from coagulase-negative staphylococci?

Two hours after a dinner of leftover fried rice from a roadside eatery in Mumbai, four college students develop nausea and repeated vomiting without fever. The rice had been boiled in the morning and kept at room temperature before frying. Which organism is most likely responsible?

A 21-year-old newly married woman in Thiruvananthapuram has burning micturition and frequency. Urine culture grows more than 100,000 CFU/mL of catalase-positive, coagulase-negative Gram-positive cocci in clusters. The isolate grows up to the edge of a novobiocin disc. Which organism is this?

A 7-year-old boy in a Malappuram ward is confirmed to have diphtheria by culture and toxigenicity testing. His 4-year-old sister, who shares his bed, is well, and her vaccination card shows only one dose of pentavalent vaccine. What should be done for the sister?

A 66-year-old man from Hyderabad has fever for three weeks. Echocardiography shows a vegetation on the aortic valve, and blood cultures grow Streptococcus gallolyticus (formerly S. bovis biotype I). He is started on IV penicillin. Which further investigation is most important for him?