During a Clostridioides difficile outbreak, staff are told to wash hands with soap and water rather than use alcohol hand rub. Why?

Alcohol does not kill the spores

Soap kills vegetative cells faster

Alcohol fails to remove the toxins

Alcohol rubs spread spores in air

In blood culture for enteric fever, clot culture with streptokinase can give better isolation than whole blood culture because:

Bactericidal factors in the serum are left out

Antibiotic in the blood is bound by the clot

Streptokinase stimulates salmonellae to divide

Clotting concentrates bacteria from plasma

The 0.5 McFarland turbidity standard is prepared by mixing:

Barium chloride and sulphuric acid

Silver nitrate and sodium chloride

Calcium chloride and sodium carbonate

Ferric chloride and potassium thiocyanate

In disk diffusion, plates should be incubated within 15 minutes of applying the disks. Leaving them on the bench for an hour gives:

Falsely large zones from pre-diffusion

Overlapping zones from disks moving

Double zones from mixed inoculum growth

Falsely small zones from drug breakdown

An infection is classed as hospital-acquired (nosocomial) when it appears:

48 hours or more after admission

Only in the intensive care unit

Only after a surgical procedure

Diagnostic bacteriology and antimicrobial susceptibility

56 Kerala MSc MLT Entrance practice questions on Diagnostic bacteriology and antimicrobial susceptibility, 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
During a Clostridioides difficile outbreak, staff are told to wash hands with soap and water rather than use alcohol hand rub. Why?
1. ASoap kills vegetative cells faster
2. BAlcohol does not kill the spores
3. CAlcohol fails to remove the toxins
4. DAlcohol rubs spread spores in air
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Answer: B · Alcohol does not kill the spores
C. difficile spreads as spores, which survive alcohol hand rubs. Washing with soap and running water removes spores mechanically from the hands. Gloves, contact precautions and cleaning surfaces with a sporicidal agent such as hypochlorite are also used. Alcohol rub remains the standard for most other organisms.
- A: Alcohol kills vegetative cells well; the problem is the spores.
- C: The concern is transmission of spores, not toxin on the hands.
- D: Alcohol rubs do not aerosolise spores.
Question 2 · Medium
In blood culture for enteric fever, clot culture with streptokinase can give better isolation than whole blood culture because:
1. AAntibiotic in the blood is bound by the clot
2. BStreptokinase stimulates salmonellae to divide
3. CBactericidal factors in the serum are left out
4. DClotting concentrates bacteria from plasma
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Answer: C · Bactericidal factors in the serum are left out
In clot culture, the serum is separated for the Widal test and the clot is digested with streptokinase and cultured in bile broth. Because the serum is removed, its antibodies and complement cannot kill the bacilli in the culture. The bacilli trapped in the clot with the cells are released by the digestion. The method makes use of a single sample for both culture and serology.
- A: The clot does not remove antibiotics; recent antibiotics reduce yield either way.
- B: Streptokinase digests the clot; it does not promote bacterial growth.
- D: Bacteria are trapped with cells, but the main gain is removing serum factors.
Question 3 · Easy
The 0.5 McFarland turbidity standard is prepared by mixing:
1. ABarium chloride and sulphuric acid
2. BSilver nitrate and sodium chloride
3. CCalcium chloride and sodium carbonate
4. DFerric chloride and potassium thiocyanate
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Answer: A · Barium chloride and sulphuric acid
The 0.5 McFarland standard is made by adding 0.05 mL of 1.175% barium chloride to 9.95 mL of 1% sulphuric acid, forming a fine barium sulphate suspension. Its turbidity matches about 1.5 x 10^8 CFU/mL of bacteria. It is stored in the dark in a sealed tube and mixed well before use. Its density is checked at 625 nm, where the absorbance should be 0.08 to 0.13.
- B: Silver nitrate and sodium chloride form silver chloride, which darkens in light.
- C: Calcium carbonate is not used for McFarland standards.
- D: Ferric thiocyanate gives a red solution, not a turbid suspension.
Question 4 · Medium
In disk diffusion, plates should be incubated within 15 minutes of applying the disks. Leaving them on the bench for an hour gives:
1. AFalsely large zones from pre-diffusion
2. BOverlapping zones from disks moving
3. CDouble zones from mixed inoculum growth
4. DFalsely small zones from drug breakdown
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Answer: A · Falsely large zones from pre-diffusion
Once disks touch the agar, the drug starts to diffuse while the organisms are not yet growing. If plates stand at room temperature, the drug spreads further before growth begins, so zones are larger and resistant strains may read susceptible. CLSI therefore advises inoculating within 15 minutes of preparing the suspension and incubating within 15 minutes of applying the disks. Plates are then incubated at 35 °C for 16 to 18 hours for most organisms.
- B: Disks do not move once pressed down.
- C: Double zones come from mixed cultures, not from delayed incubation.
- D: The drug is stable for an hour at room temperature; the problem is extra diffusion.
Question 5 · Easy
An infection is classed as hospital-acquired (nosocomial) when it appears:
1. AWithin 12 hours of admission
2. B48 hours or more after admission
3. COnly in the intensive care unit
4. DOnly after a surgical procedure
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Answer: B · 48 hours or more after admission
An infection that was not present or incubating at admission and appears 48 hours or more after admission is regarded as hospital-acquired. Infections appearing soon after discharge, and surgical site infections within 30 days of surgery, are also counted. Common types are urinary tract infections, pneumonia, surgical site infections and bloodstream infections. Hand hygiene is the single most important preventive measure.
- A: An infection within 12 hours was almost certainly incubating at admission.
- C: Hospital infections occur on all wards, not only in ICUs.
- D: Hospital infections include non-surgical types such as catheter UTIs.
Question 6 · Easy
By CLSI, the centres of antibiotic disks on a Mueller-Hinton plate must be at least how far apart?
1. A15 mm
2. B10 mm
3. C24 mm
4. D40 mm
Answer and explanation in LearnFree account
Question 7 · Medium
Why are refrigerated antibiotic disk cartridges allowed to reach room temperature before they are opened?
1. ATo let the antibiotic dissolve into the disk
2. BTo prevent condensation that degrades the drug
3. CTo activate the drug before it touches agar
4. DTo make the disks stick firmly to the agar
Answer and explanation in LearnFree account
Question 8 · Hard
An Enterobacter cloacae isolate is tested by disk diffusion. The ceftazidime zone is flattened on the side facing a nearby cefoxitin disk, while the rest of the zone is a normal circle. What does this show?
1. AInducible chromosomal AmpC beta-lactamase
2. BExtended-spectrum beta-lactamase
3. CDisks placed too close together
4. DA metallo-beta-lactamase
Answer and explanation in LearnFree account
Question 9 · Medium
A 35-year-old woman has frequency, dysuria and persistent pus cells in acidic urine, but routine urine cultures are repeatedly sterile. Which infection should be looked for?
1. ARenal tuberculosis
2. BStaphylococcus saprophyticus cystitis
3. CProteus urinary infection
4. DAcute pyelonephritis
Answer and explanation in LearnFree account
Question 10 · Hard
A patient on long-term parenteral nutrition through a central line has fever. Paired blood cultures drawn at the same time grow Staphylococcus epidermidis: the catheter bottle flags positive at 9 hours and the peripheral bottle at 13 hours. How should this be interpreted?
1. ABacteraemia from a source other than the line
2. BColonisation of the line without bacteraemia
3. CSkin contamination of the peripheral set
4. DCatheter-related bloodstream infection
Answer and explanation in LearnFree account
Question 11 · Hard
In a tube dilution test, 1 mL of antibiotic is diluted twofold in broth from 128 µg/mL (tube 1) to 1 µg/mL (tube 8). Then 1 mL of standard inoculum is added to each tube. After overnight incubation, tubes 1 to 5 are clear and tubes 6 to 8 are turbid. What is the MIC?
1. A2 µg/mL
2. B16 µg/mL
3. C8 µg/mL
4. D4 µg/mL
Answer and explanation in LearnFree account
Question 12 · Medium
In the imipenem-EDTA combined disk test on a carbapenem-resistant isolate, a zone increase of 7 mm or more around the EDTA disk indicates:
1. AA metallo-beta-lactamase
2. BAn AmpC beta-lactamase
3. CAn OXA-48-like enzyme
4. DA KPC carbapenemase
Answer and explanation in LearnFree account
Question 13 · Medium
In the CLSI phenotypic confirmatory test for ESBL in Escherichia coli, which result confirms ESBL production?
1. AResistance to cefoxitin as well as ceftazidime
2. BNo change in zone size with clavulanate added
3. CA zone increase of 5 mm or more with clavulanate
4. DFlattening of a zone next to an imipenem disk
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Question 14 · Hard
A Gram-stained vaginal smear is scored by Nugent's method: Lactobacillus morphotypes 1+, Gardnerella/Bacteroides morphotypes 4+, curved gram-variable rods 2+. What are the score and interpretation?
1. A5: intermediate flora
2. B9: bacterial vaginosis
3. C6: intermediate flora
4. D8: bacterial vaginosis
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Question 15 · Medium
An Enterococcus faecalis from endocarditis shows no zone around a high-content gentamicin (120 µg) disk. What does this result predict?
1. AAmpicillin plus gentamicin will not act synergistically
2. BThe isolate is also resistant to ampicillin
3. CThe isolate must carry the vanA gene
4. DGentamicin alone will cure the endocarditis
Answer and explanation in LearnFree account
Question 16 · Hard
A 28-year-old man has purulent urethral discharge. Gram stain shows gram-negative diplococci inside pus cells, and the isolate on modified Thayer-Martin agar is nitrocefin positive. What does the nitrocefin result mean for treatment?
1. AThe isolate is resistant to all beta-lactams
2. BPenicillin will fail; ceftriaxone is needed
3. CCeftriaxone will fail; penicillin is needed
4. DThe isolate needs colistin for treatment
Answer and explanation in LearnFree account
Question 17 · Hard
A patient with a urinary catheter for 10 days has fever. A nurse sends urine poured from the drainage bag. Culture grows three different gram-negative bacilli, each over 10^5 CFU/mL. What is the correct action?
1. AReport all three organisms with full susceptibility
2. BReport the organism with the highest colony count
3. CRequest urine aspirated from the catheter sampling port
4. DRepeat culture from the same bag urine after 24 hours
Answer and explanation in LearnFree account
Question 18 · Medium
Which specimen is suitable for anaerobic culture?
1. AMidstream urine from a woman with pyelonephritis
2. BSwab of a superficial leg ulcer surface
3. CExpectorated sputum collected in the morning
4. DPus aspirated by syringe from a deep abscess
Answer and explanation in LearnFree account
Question 19 · Hard
A boil isolate of Staphylococcus aureus grows right up to an erythromycin disk. Next to it, the clindamycin disk (18 mm away) shows a perfectly round 25 mm zone with no blunting on any side. Which mechanism and clindamycin report fit best?
1. AConstitutive erm; clindamycin resistant
2. BDisks too far apart; test invalid
3. CmsrA efflux; clindamycin susceptible
4. DInducible erm; clindamycin resistant
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Question 20 · Medium
Two sets of blood cultures are drawn from a febrile 60-year-old without any line or prosthesis. Only one aerobic bottle grows coagulase-negative staphylococci, after 3 days. How should this most likely be interpreted?
1. AAs a probable skin contaminant
2. BAs true bacteraemia needing therapy
3. CAs a sign of infective endocarditis
4. DAs proof of methicillin resistance
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Question 21 · Medium
In suspected ventilator-associated pneumonia, quantitative culture of bronchoalveolar lavage fluid is usually taken as significant at a count of at least:
1. A10^4 CFU/mL
2. B10^3 CFU/mL
3. C10^2 CFU/mL
4. D10^7 CFU/mL
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Question 22 · Medium
An Enterococcus faecium from a blood culture has a vancomycin MIC of 256 µg/mL and a teicoplanin MIC of 64 µg/mL. Which resistance genotype fits best?
1. AvanC
2. BmecA
3. CvanB
4. DvanA
Answer and explanation in LearnFree account
Question 23 · Medium
A sputum sent for routine culture shows more than 25 squamous epithelial cells and 3 pus cells per low-power field on Gram stain. What should the laboratory do?
1. ACulture it and report all organisms grown
2. BAsk for a fresh specimen, as this is saliva
3. CCulture it only for Mycobacterium tuberculosis
4. DProcess it after washing it in sterile saline
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Question 24 · Easy
When a gradient strip (E-test) is read after overnight incubation, the MIC is the value printed on the strip where:
1. AThe strip's highest concentration has been printed
2. BThe widest part of the zone of inhibition lies
3. CThe edge of the elliptical zone meets the strip
4. DThe first isolated colonies appear inside the zone
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Question 25 · Hard
Within one month, four patients operated on in the same theatre develop wound infections with MRSA of identical antibiogram. Theatre sterilisation records are normal. Which investigation is most useful to find the source?
1. ASwabs from every ward wall and floor
2. BRepeat culture of the autoclaved instruments
3. CAnterior nasal swabs from the theatre staff
4. DSettle plates from the theatre air only
Answer and explanation in LearnFree account

31 more Diagnostic bacteriology and antimicrobial susceptibility questions

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Study Microbiology

Diagnostic bacteriology and antimicrobial susceptibility

During a Clostridioides difficile outbreak, staff are told to wash hands with soap and water rather than use alcohol hand rub. Why?

In blood culture for enteric fever, clot culture with streptokinase can give better isolation than whole blood culture because:

The 0.5 McFarland turbidity standard is prepared by mixing:

In disk diffusion, plates should be incubated within 15 minutes of applying the disks. Leaving them on the bench for an hour gives:

An infection is classed as hospital-acquired (nosocomial) when it appears:

By CLSI, the centres of antibiotic disks on a Mueller-Hinton plate must be at least how far apart?

Why are refrigerated antibiotic disk cartridges allowed to reach room temperature before they are opened?

An Enterobacter cloacae isolate is tested by disk diffusion. The ceftazidime zone is flattened on the side facing a nearby cefoxitin disk, while the rest of the zone is a normal circle. What does this show?

A 35-year-old woman has frequency, dysuria and persistent pus cells in acidic urine, but routine urine cultures are repeatedly sterile. Which infection should be looked for?

A patient on long-term parenteral nutrition through a central line has fever. Paired blood cultures drawn at the same time grow Staphylococcus epidermidis: the catheter bottle flags positive at 9 hours and the peripheral bottle at 13 hours. How should this be interpreted?

In a tube dilution test, 1 mL of antibiotic is diluted twofold in broth from 128 µg/mL (tube 1) to 1 µg/mL (tube 8). Then 1 mL of standard inoculum is added to each tube. After overnight incubation, tubes 1 to 5 are clear and tubes 6 to 8 are turbid. What is the MIC?

In the imipenem-EDTA combined disk test on a carbapenem-resistant isolate, a zone increase of 7 mm or more around the EDTA disk indicates:

In the CLSI phenotypic confirmatory test for ESBL in Escherichia coli, which result confirms ESBL production?

A Gram-stained vaginal smear is scored by Nugent's method: Lactobacillus morphotypes 1+, Gardnerella/Bacteroides morphotypes 4+, curved gram-variable rods 2+. What are the score and interpretation?

An Enterococcus faecalis from endocarditis shows no zone around a high-content gentamicin (120 µg) disk. What does this result predict?

A 28-year-old man has purulent urethral discharge. Gram stain shows gram-negative diplococci inside pus cells, and the isolate on modified Thayer-Martin agar is nitrocefin positive. What does the nitrocefin result mean for treatment?

A patient with a urinary catheter for 10 days has fever. A nurse sends urine poured from the drainage bag. Culture grows three different gram-negative bacilli, each over 10^5 CFU/mL. What is the correct action?

Which specimen is suitable for anaerobic culture?

A boil isolate of Staphylococcus aureus grows right up to an erythromycin disk. Next to it, the clindamycin disk (18 mm away) shows a perfectly round 25 mm zone with no blunting on any side. Which mechanism and clindamycin report fit best?

Two sets of blood cultures are drawn from a febrile 60-year-old without any line or prosthesis. Only one aerobic bottle grows coagulase-negative staphylococci, after 3 days. How should this most likely be interpreted?

In suspected ventilator-associated pneumonia, quantitative culture of bronchoalveolar lavage fluid is usually taken as significant at a count of at least:

An Enterococcus faecium from a blood culture has a vancomycin MIC of 256 µg/mL and a teicoplanin MIC of 64 µg/mL. Which resistance genotype fits best?

A sputum sent for routine culture shows more than 25 squamous epithelial cells and 3 pus cells per low-power field on Gram stain. What should the laboratory do?

When a gradient strip (E-test) is read after overnight incubation, the MIC is the value printed on the strip where:

Within one month, four patients operated on in the same theatre develop wound infections with MRSA of identical antibiogram. Theatre sterilisation records are normal. Which investigation is most useful to find the source?