Shigella: biochemical features: 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 · Shigella: biochemical features

Sample item

A child has bloody diarrhoea. Stool culture on MacConkey agar grows non-motile colonies that are pale at 18 hours but turn pink after 48 hours. The isolate is ONPG positive and ornithine decarboxylase positive. Which organism is it?

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

The answer and the reasoning

B. Shigella sonnei

Shigella sonnei is a late lactose fermenter that is ONPG positive and ornithine decarboxylase positive, unlike other Shigella species. It is non-motile and does not produce H2S. Most Shigella are non-lactose fermenters.

Option by option

  • A. Shigella flexneri

    S. flexneri does not ferment lactose and is ornithine decarboxylase negative.

  • B. Shigella sonnei (correct)

    Correct. Late lactose fermentation, positive ONPG and ornithine decarboxylase characterise S. sonnei.

  • C. Shigella dysenteriae type 1

    S. dysenteriae is a non-lactose fermenter and is ornithine decarboxylase negative.

  • D. Salmonella Typhi

    Salmonella Typhi is motile and produces H2S.

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 B, Shigella sonnei. Shigella sonnei is a late lactose fermenter that is ONPG positive and ornithine decarboxylase positive, unlike other Shigella species.

Why is "Shigella flexneri" not the answer?

S. flexneri does not ferment lactose and is ornithine decarboxylase negative.

Why is "Shigella dysenteriae type 1" not the answer?

S. dysenteriae is a non-lactose fermenter and is ornithine decarboxylase negative.

Why is "Salmonella Typhi" not the answer?

Salmonella Typhi is motile and produces H2S.

What concept does this question test?

It tests Shigella: biochemical features in Microbiology. It is non-motile and does not produce H2S. Most Shigella are non-lactose fermenters.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Shigella: biochemical features?

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.