General microbiology and sterilisation: Ames 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.

Try it

Microbiology · General microbiology and sterilisation: Ames test

Sample item

A food laboratory screens a new additive by plating histidine-requiring mutant Salmonella on histidine-free agar, with a rat liver extract added to the additive. A markedly increased number of colonies appear compared with the control. What does this indicate?

AThe additive is bactericidalBThe additive is a source of histidineCThe additive or its metabolite is mutagenicDThe additive is sterile

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

The answer and the reasoning

C. The additive or its metabolite is mutagenic

In the Ames test, auxotrophic strains that cannot make histidine revert to prototrophy if a chemical causes mutation. The revertant colonies grow without histidine, so more colonies than the control indicates mutagenicity, and mutagens are often potential carcinogens. Liver extract supplies metabolic activation of pro-mutagens.

Option by option

A. The additive is bactericidal
A bactericidal substance would reduce growth or create a clear zone, not increase colony numbers.
B. The additive is a source of histidine
A histidine source would let the auxotrophs grow without any mutation, which is a confounder in the assay and not the meaning of a positive Ames result.
C. The additive or its metabolite is mutagenic (correct)
Correct. Increased revertants that grow without histidine signal mutagenic activity.
D. The additive is sterile
Sterility of the additive has no bearing on the growth of revertant colonies.

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.

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is C, The additive or its metabolite is mutagenic. In the Ames test, auxotrophic strains that cannot make histidine revert to prototrophy if a chemical causes mutation.

Why is "The additive is bactericidal" not the answer?

A bactericidal substance would reduce growth or create a clear zone, not increase colony numbers.

Why is "The additive is a source of histidine" not the answer?

A histidine source would let the auxotrophs grow without any mutation, which is a confounder in the assay and not the meaning of a positive Ames result.

Why is "The additive is sterile" not the answer?

Sterility of the additive has no bearing on the growth of revertant colonies.

What concept does this question test?

It tests General microbiology and sterilisation: Ames test in Microbiology. The revertant colonies grow without histidine, so more colonies than the control indicates mutagenicity, and mutagens are often potential carcinogens. Liver extract supplies metabolic activation of pro-mutagens.

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: Ames 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.