General microbiology and sterilisation: infective dose: 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: infective dose

Sample item

Which of the following enteric pathogens is known for the lowest infective dose by the oral route?

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

The answer and the reasoning

A. Shigella dysenteriae

Shigella can cause disease with only about 10 to 100 organisms because it is relatively acid resistant. Vibrio cholerae, non-typhoidal Salmonella and enterotoxigenic E. coli usually need much larger inocula, often in the order of millions or more. Low infective dose explains why shigellosis spreads easily by person-to-person contact.

Option by option

  • A. Shigella dysenteriae (correct)

    Correct. Shigella needs only a few tens to hundreds of organisms to cause dysentery.

  • B. Salmonella Typhimurium

    Non-typhoidal Salmonella generally needs a much larger dose than Shigella.

  • C. Enterotoxigenic Escherichia coli

    Enterotoxigenic E. coli diarrhoea usually follows a large inoculum in contaminated food or water.

  • D. Vibrio cholerae

    Vibrio cholerae is acid sensitive and needs a very high inoculum unless gastric acidity is reduced.

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, Shigella dysenteriae. Shigella can cause disease with only about 10 to 100 organisms because it is relatively acid resistant.

Why is "Salmonella Typhimurium" not the answer?

Non-typhoidal Salmonella generally needs a much larger dose than Shigella.

Why is "Enterotoxigenic Escherichia coli" not the answer?

Enterotoxigenic E. coli diarrhoea usually follows a large inoculum in contaminated food or water.

Why is "Vibrio cholerae" not the answer?

Vibrio cholerae is acid sensitive and needs a very high inoculum unless gastric acidity is reduced.

What concept does this question test?

It tests General microbiology and sterilisation: infective dose in Microbiology. Vibrio cholerae, non-typhoidal Salmonella and enterotoxigenic E. coli usually need much larger inocula, often in the order of millions or more. Low infective dose explains why shigellosis spreads easily by person-to-person contact.

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: infective dose?

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.