Culture media: TCBS agar: 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 · Culture media: TCBS agar

Sample item

A man develops acute watery diarrhoea after eating raw shellfish. Stool plated on TCBS agar grows green colonies. Which organism is most likely?

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

The answer and the reasoning

A. Vibrio parahaemolyticus

TCBS agar contains sucrose and a bromothymol blue indicator. Sucrose fermenters such as V. cholerae and V. alginolyticus produce acid and yellow colonies, while non-fermenters such as V. parahaemolyticus give green or blue-green colonies. Raw seafood diarrhoea points to V. parahaemolyticus.

Option by option

  • A. Vibrio parahaemolyticus (correct)

    Correct. A sucrose non-fermenter with a raw seafood history fits V. parahaemolyticus.

  • B. Vibrio cholerae

    V. cholerae ferments sucrose and forms yellow colonies on TCBS.

  • C. Vibrio alginolyticus

    V. alginolyticus ferments sucrose and gives yellow colonies; it more often causes ear and wound infection.

  • D. Escherichia coli

    E. coli is inhibited by the high pH, bile salts and thiosulfate in TCBS.

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.

More Microbiology questions

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is A, Vibrio parahaemolyticus. TCBS agar contains sucrose and a bromothymol blue indicator.

Why is "Vibrio cholerae" not the answer?

V. cholerae ferments sucrose and forms yellow colonies on TCBS.

Why is "Vibrio alginolyticus" not the answer?

V. alginolyticus ferments sucrose and gives yellow colonies; it more often causes ear and wound infection.

Why is "Escherichia coli" not the answer?

E. coli is inhibited by the high pH, bile salts and thiosulfate in TCBS.

What concept does this question test?

It tests Culture media: TCBS agar in Microbiology. Sucrose fermenters such as V. cholerae and V. alginolyticus produce acid and yellow colonies, while non-fermenters such as V. parahaemolyticus give green or blue-green colonies. Raw seafood diarrhoea points to V. parahaemolyticus.

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 Culture media: TCBS agar?

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.