General microbiology: selective media: 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: selective media

Sample item

A stool culture from a suspected typhoid carrier among food handlers is plated on a bismuth sulphite-based selective medium. After 48 hours the laboratory sees jet-black colonies with a metallic sheen. Which medium was used?

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

The answer and the reasoning

D. Wilson-Blair medium

Wilson-Blair medium is a bismuth sulphite agar in which Salmonella Typhi reduces sulphite to hydrogen sulphide, forming black colonies with a metallic sheen. Brilliant green and bismuth suppress most coliforms. It is a selective medium for isolating Salmonella Typhi from faeces.

Option by option

  • A. Thayer-Martin medium

    Thayer-Martin is a selective chocolate agar with antibiotics for Neisseria gonorrhoeae.

  • B. Loeffler serum slope

    Loeffler serum slope enhances the growth and granule formation of Corynebacterium diphtheriae.

  • C. Potassium tellurite blood agar

    Tellurite blood agar is selective for Corynebacterium diphtheriae, which gives grey-black colonies.

  • D. Wilson-Blair medium (correct)

    Correct. Bismuth sulphite with H2S production gives black colonies of Salmonella Typhi.

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.

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Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is D, Wilson-Blair medium. Wilson-Blair medium is a bismuth sulphite agar in which Salmonella Typhi reduces sulphite to hydrogen sulphide, forming black colonies with a metallic sheen.

Why is "Thayer-Martin medium" not the answer?

Thayer-Martin is a selective chocolate agar with antibiotics for Neisseria gonorrhoeae.

Why is "Loeffler serum slope" not the answer?

Loeffler serum slope enhances the growth and granule formation of Corynebacterium diphtheriae.

Why is "Potassium tellurite blood agar" not the answer?

Tellurite blood agar is selective for Corynebacterium diphtheriae, which gives grey-black colonies.

What concept does this question test?

It tests General microbiology: selective media in Microbiology. Brilliant green and bismuth suppress most coliforms. It is a selective medium for isolating Salmonella Typhi from faeces.

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: selective media?

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.