General microbiology and sterilisation: Gram stain artefacts: 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: Gram stain artefacts

Sample item

A Gram stain from a 72-hour culture of a known Gram-positive Bacillus shows many pink rods alongside a few purple ones. What is the most likely reason?

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

The answer and the reasoning

D. Cell wall damage in ageing cells

Old cultures contain cells with degraded peptidoglycan, which cannot retain the crystal violet-iodine complex, so they appear Gram variable or negative. Always Gram stain young cultures. Omitting the decolouriser would leave all cells purple.

Option by option

  • A. Excess iodine mordant

    More iodine would strengthen, not weaken, retention of the complex.

  • B. Overuse of the primary stain

    Excess primary stain would tend to keep cells purple.

  • C. Omission of the decolouriser

    Omitting the decolouriser would leave everything purple, not pink.

  • D. Cell wall damage in ageing cells (correct)

    Correct. Autolysis in aged cells lets the dye complex wash out.

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, Cell wall damage in ageing cells. Old cultures contain cells with degraded peptidoglycan, which cannot retain the crystal violet-iodine complex, so they appear Gram variable or negative.

Why is "Excess iodine mordant" not the answer?

More iodine would strengthen, not weaken, retention of the complex.

Why is "Overuse of the primary stain" not the answer?

Excess primary stain would tend to keep cells purple.

Why is "Omission of the decolouriser" not the answer?

Omitting the decolouriser would leave everything purple, not pink.

What concept does this question test?

It tests General microbiology and sterilisation: Gram stain artefacts in Microbiology. Always Gram stain young cultures. Omitting the decolouriser would leave all cells purple.

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: Gram stain artefacts?

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.