General microbiology and sterilisation: viable count calculation: 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: viable count calculation

Sample item

A broth culture is serially diluted, and 0.1 mL of the 10^-5 dilution is spread on an agar plate, giving 120 colonies after incubation. What is the viable count of the original culture per mL?

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

The answer and the reasoning

A. 1.2 x 10^8

Colonies per mL of the diluted sample equal 120 divided by 0.1, which is 1200 per mL. Multiplying by the reciprocal of the dilution (10^5) gives 1.2 x 10^8 colony-forming units per mL in the original culture. Always include the plated volume in the calculation.

Option by option

  • A. 1.2 x 10^8 (correct)

    Correct. 120 divided by 0.1 mL gives 1200 per mL, and multiplying by 10^5 gives 1.2 x 10^8.

  • B. 1.2 x 10^7

    This omits the correction for plating only 0.1 mL.

  • C. 1.2 x 10^6

    This uses the dilution factor but forgets to convert the 0.1 mL plated volume to per mL.

  • D. 1.2 x 10^9

    This arises from wrongly multiplying by an extra factor of 10.

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, 1.2 x 10^8. Colonies per mL of the diluted sample equal 120 divided by 0.

Why is "1.2 x 10^7" not the answer?

This omits the correction for plating only 0.1 mL.

Why is "1.2 x 10^6" not the answer?

This uses the dilution factor but forgets to convert the 0.1 mL plated volume to per mL.

Why is "1.2 x 10^9" not the answer?

This arises from wrongly multiplying by an extra factor of 10.

What concept does this question test?

It tests General microbiology and sterilisation: viable count calculation in Microbiology. 1, which is 1200 per mL. Multiplying by the reciprocal of the dilution (10^5) gives 1.2 x 10^8 colony-forming units per mL in the original culture. Always include the plated volume in the calculation.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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: viable count calculation?

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.