General microbiology and sterilisation: decimal reduction time: 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: decimal reduction time

Sample item

A spore suspension contains 10^6 viable spores per mL, and its decimal reduction time at 121 degrees C is 1.5 minutes. Assuming log-linear killing, how long is needed to bring the count down to 1 spore per mL?

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

The answer and the reasoning

D. 9 minutes

The D value is the time needed at a given temperature to kill 90 percent of the population, which is a one log reduction. Reducing 10^6 to 10^0 needs six log reductions, and 6 multiplied by 1.5 minutes is 9 minutes. In practice, sterilisation cycles add a safety margin well beyond this.

Option by option

  • A. 4 minutes

    This underestimates the time, since six log reductions are required rather than four.

  • B. 6 minutes

    This is the number of log reductions, with the D value ignored.

  • C. 15 minutes

    This would apply only if each log reduction took 2.5 minutes, which is not the stated D value.

  • D. 9 minutes (correct)

    Correct. Six log10 reductions at 1.5 minutes each give 9 minutes.

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 D, 9 minutes. The D value is the time needed at a given temperature to kill 90 percent of the population, which is a one log reduction.

Why is "4 minutes" not the answer?

This underestimates the time, since six log reductions are required rather than four.

Why is "6 minutes" not the answer?

This is the number of log reductions, with the D value ignored.

Why is "15 minutes" not the answer?

This would apply only if each log reduction took 2.5 minutes, which is not the stated D value.

What concept does this question test?

It tests General microbiology and sterilisation: decimal reduction time in Microbiology. Reducing 10^6 to 10^0 needs six log reductions, and 6 multiplied by 1.5 minutes is 9 minutes. In practice, sterilisation cycles add a safety margin well beyond this.

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: decimal reduction time?

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.