General microbiology and sterilisation: alkaline peptone water: practice question

A Microbiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Microbiology · General microbiology and sterilisation: alkaline peptone water

Sample item

A stool sample from a patient with suspected cholera is first inoculated into alkaline peptone water (pH about 8.6) and incubated for a few hours before subculture to a selective plate. What is the rationale?

AThe alkaline pH is needed for vibrios to form sporesBVibrios multiply rapidly at alkaline pH while most other faecal flora are suppressedCThe alkaline pH destroys the cholera toxin to make the sample safeDThe alkaline broth lowers the oxygen tension for anaerobic growth

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Vibrios multiply rapidly at alkaline pH while most other faecal flora are suppressed

Vibrio cholerae tolerates alkaline conditions, so alkaline peptone water enriches it relative to other intestinal bacteria. The surface layer of the broth is usually subcultured after about 6 to 8 hours, before overgrowth by competitors. The enriched growth is then plated on TCBS agar.

Option by option

A. The alkaline pH is needed for vibrios to form spores
Vibrio cholerae does not form spores.
B. Vibrios multiply rapidly at alkaline pH while most other faecal flora are suppressed (correct)
Correct. Alkaline pH favours the vibrios, enriching them before plating.
C. The alkaline pH destroys the cholera toxin to make the sample safe
The purpose is to grow the organism, not to inactivate toxin.
D. The alkaline broth lowers the oxygen tension for anaerobic growth
Vibrio is a facultative organism, and the enrichment depends on pH, not on lowering oxygen.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is B, Vibrios multiply rapidly at alkaline pH while most other faecal flora are suppressed. Vibrio cholerae tolerates alkaline conditions, so alkaline peptone water enriches it relative to other intestinal bacteria.

Why is "The alkaline pH is needed for vibrios to form spores" not the answer?

Vibrio cholerae does not form spores.

Why is "The alkaline pH destroys the cholera toxin to make the sample safe" not the answer?

The purpose is to grow the organism, not to inactivate toxin.

Why is "The alkaline broth lowers the oxygen tension for anaerobic growth" not the answer?

Vibrio is a facultative organism, and the enrichment depends on pH, not on lowering oxygen.

What concept does this question test?

It tests General microbiology and sterilisation: alkaline peptone water in Microbiology. The surface layer of the broth is usually subcultured after about 6 to 8 hours, before overgrowth by competitors. The enriched growth is then plated on TCBS agar.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study General microbiology and sterilisation: alkaline peptone water?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.