General microbiology and sterilisation: oxidation-fermentation test: 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: oxidation-fermentation test

Sample item

A Gram-negative bacillus is inoculated into two tubes of glucose semisolid medium, and one tube is sealed with liquid paraffin. After incubation, acid production turns only the open tube yellow. Which organism fits this result?

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

The answer and the reasoning

A. Pseudomonas aeruginosa

In the Hugh-Leifson oxidation-fermentation test, an oxidiser makes acid only in the aerobic (open) tube, whereas a fermenter makes acid in both tubes. Pseudomonas aeruginosa is a strict aerobic oxidiser of glucose. Members of Enterobacterales and Vibrio are fermenters.

Option by option

  • A. Pseudomonas aeruginosa (correct)

    Correct. Acid only under aerobic conditions indicates oxidative metabolism, as in Pseudomonas.

  • B. Klebsiella pneumoniae

    Klebsiella is a fermenter and acidifies both tubes.

  • C. Escherichia coli

    E. coli ferments glucose, so both the open and the sealed tubes turn yellow.

  • D. Vibrio cholerae

    Vibrio cholerae ferments glucose fermentatively, so the sealed tube also turns yellow.

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, Pseudomonas aeruginosa. In the Hugh-Leifson oxidation-fermentation test, an oxidiser makes acid only in the aerobic (open) tube, whereas a fermenter makes acid in both tubes.

Why is "Klebsiella pneumoniae" not the answer?

Klebsiella is a fermenter and acidifies both tubes.

Why is "Escherichia coli" not the answer?

E. coli ferments glucose, so both the open and the sealed tubes turn yellow.

Why is "Vibrio cholerae" not the answer?

Vibrio cholerae ferments glucose fermentatively, so the sealed tube also turns yellow.

What concept does this question test?

It tests General microbiology and sterilisation: oxidation-fermentation test in Microbiology. Pseudomonas aeruginosa is a strict aerobic oxidiser of glucose. Members of Enterobacterales and Vibrio are fermenters.

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: oxidation-fermentation test?

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.