Bacterial growth: generation time 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 · Bacterial growth: generation time calculation

Sample item

Escherichia coli has a generation time of 20 minutes in log phase. If 2 x 10^3 cells in log phase are inoculated into broth and lag is ignored, what is the approximate count after 3 hours?

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

The answer and the reasoning

C. 1.0 x 10^6

Three hours is 180 minutes, so there are 180 / 20 = 9 generations. Growth is exponential: N = N0 x 2^n = 2 x 10^3 x 2^9 = 2000 x 512 = 1,024,000, or about 1.0 x 10^6. Using 2^3 or multiplying by 180 are common errors.

Option by option

  • A. 3.6 x 10^5

    This treats growth as linear (2000 x 180), but bacteria multiply geometrically.

  • B. 1.6 x 10^4

    This uses 2^3 (three hours treated as three generations), which ignores the 9 doublings.

  • C. 1.0 x 10^6 (correct)

    Correct. 2000 x 512 is about 1.0 x 10^6.

  • D. 5.1 x 10^5

    This uses 1000 x 512 and forgets the starting inoculum of 2000 cells.

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.

More Microbiology questions

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is C, 1.0 x 10^6. Three hours is 180 minutes, so there are 180 / 20 = 9 generations.

Why is "3.6 x 10^5" not the answer?

This treats growth as linear (2000 x 180), but bacteria multiply geometrically.

Why is "1.6 x 10^4" not the answer?

This uses 2^3 (three hours treated as three generations), which ignores the 9 doublings.

Why is "5.1 x 10^5" not the answer?

This uses 1000 x 512 and forgets the starting inoculum of 2000 cells.

What concept does this question test?

It tests Bacterial growth: generation time calculation in Microbiology. Growth is exponential: N = N0 x 2^n = 2 x 10^3 x 2^9 = 2000 x 512 = 1,024,000, or about 1.0 x 10^6. Using 2^3 or multiplying by 180 are common errors.

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 Bacterial growth: generation time 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.