Bacteriology: Mycobacterium marinum: 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 · Bacteriology: Mycobacterium marinum

Sample item

A 35-year-old aquarium hobbyist develops violaceous nodules in a linear pattern up the forearm several weeks after cleaning a fish tank with an abraded finger. Acid-fast bacilli are seen in a biopsy. At which temperature should Lowenstein-Jensen cultures be incubated to maximise isolation?

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

The answer and the reasoning

A. 30 to 32 degrees C

Mycobacterium marinum causes fish-tank or swimming-pool granuloma, and it grows best at 30 to 32 degrees C, which restricts the lesions to cooler body sites such as the limbs. It is a photochromogen, and incubation at 37 degrees C alone may fail to grow it. Treatment is with a combination such as clarithromycin with ethambutol or rifampicin for several months.

Option by option

  • A. 30 to 32 degrees C (correct)

    Correct. Cooler incubation matches its optimum and its cutaneous predilection.

  • B. 42 degrees C

    Thermophilic incubation at 42 degrees C would kill rather than grow this organism; it suits Campylobacter.

  • C. 37 degrees C only

    Incubation at 37 degrees C alone suits M. tuberculosis but may not grow M. marinum, which is a cool-temperature species.

  • D. 4 degrees C

    Refrigerator temperature is for storage and cold enrichment of some organisms, and mycobacteria do not multiply at 4 degrees C.

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 A, 30 to 32 degrees C. Mycobacterium marinum causes fish-tank or swimming-pool granuloma, and it grows best at 30 to 32 degrees C, which restricts the lesions to cooler body sites such as the limbs.

Why is "42 degrees C" not the answer?

Thermophilic incubation at 42 degrees C would kill rather than grow this organism; it suits Campylobacter.

Why is "37 degrees C only" not the answer?

Incubation at 37 degrees C alone suits M. tuberculosis but may not grow M. marinum, which is a cool-temperature species.

Why is "4 degrees C" not the answer?

Refrigerator temperature is for storage and cold enrichment of some organisms, and mycobacteria do not multiply at 4 degrees C.

What concept does this question test?

It tests Bacteriology: Mycobacterium marinum in Microbiology. It is a photochromogen, and incubation at 37 degrees C alone may fail to grow it. Treatment is with a combination such as clarithromycin with ethambutol or rifampicin for several months.

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 Bacteriology: Mycobacterium marinum?

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.