General microbiology and sterilisation: mannitol salt 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 · General microbiology and sterilisation: mannitol salt agar

Sample item

A nasal swab from a surgeon is plated on a medium containing 7.5 percent sodium chloride, mannitol and phenol red. Small colonies surrounded by a yellow zone grow. Which organism is most likely?

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

The answer and the reasoning

D. Staphylococcus aureus, a halotolerant mannitol fermenter

Mannitol salt agar is selective for staphylococci because of the high salt content and differential because S. aureus ferments mannitol, producing acid that turns phenol red yellow. S. epidermidis grows but leaves the medium red.

Option by option

  • A. Streptococcus pyogenes, a beta-haemolytic coccus

    S. pyogenes is not halotolerant and does not grow on this medium.

  • B. Escherichia coli, a lactose-fermenting enteric bacillus

    E. coli is inhibited by the high salt concentration and does not grow.

  • C. Staphylococcus epidermidis, a halotolerant non-fermenter of mannitol

    S. epidermidis tolerates salt but does not ferment mannitol, so the zone stays red.

  • D. Staphylococcus aureus, a halotolerant mannitol fermenter (correct)

    Correct. Growth in 7.5 percent salt with mannitol fermentation indicates S. aureus.

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, Staphylococcus aureus, a halotolerant mannitol fermenter. Mannitol salt agar is selective for staphylococci because of the high salt content and differential because S.

Why is "Streptococcus pyogenes, a beta-haemolytic coccus" not the answer?

S. pyogenes is not halotolerant and does not grow on this medium.

Why is "Escherichia coli, a lactose-fermenting enteric bacillus" not the answer?

E. coli is inhibited by the high salt concentration and does not grow.

Why is "Staphylococcus epidermidis, a halotolerant non-fermenter of mannitol" not the answer?

S. epidermidis tolerates salt but does not ferment mannitol, so the zone stays red.

What concept does this question test?

It tests General microbiology and sterilisation: mannitol salt agar in Microbiology. aureus ferments mannitol, producing acid that turns phenol red yellow. S. epidermidis grows but leaves the medium red.

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: mannitol salt 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.