General microbiology: staining: 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: staining

Sample item

A laboratory technician leaves the decolouriser on a Gram-stained smear of Staphylococcus aureus for several minutes. Under the microscope the cocci appear pink-red. What is the explanation?

AIodine was omitted, which converts the organism into a Gram-negative cellBOver-decolourisation washed crystal violet-iodine complex out of the thick wallCThe cells were stained with safranin before crystal violetDStaphylococci lack peptidoglycan and cannot hold crystal violet

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

The answer and the reasoning

B. Over-decolourisation washed crystal violet-iodine complex out of the thick wall

Decolourisation with alcohol or acetone is the critical step of the Gram stain. Prolonged exposure dissolves the crystal violet-iodine complex even from Gram-positive walls, and the counterstain safranin then colours them pink. Gram-positive bacteria have a thick peptidoglycan layer, which normally traps the complex.

Option by option

A. Iodine was omitted, which converts the organism into a Gram-negative cell
Omitting iodine would prevent retention of the dye but is not what the scenario describes.
B. Over-decolourisation washed crystal violet-iodine complex out of the thick wall (correct)
Correct. Excess decolourisation produces false Gram-negative appearance in Gram-positive cells.
C. The cells were stained with safranin before crystal violet
The staining sequence has crystal violet first; reversing it is not what happened here.
D. Staphylococci lack peptidoglycan and cannot hold crystal violet
Staphylococci have abundant peptidoglycan in the wall.

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.

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is B, Over-decolourisation washed crystal violet-iodine complex out of the thick wall. Decolourisation with alcohol or acetone is the critical step of the Gram stain.

Why is "Iodine was omitted, which converts the organism into a Gram-negative…" not the answer?

Omitting iodine would prevent retention of the dye but is not what the scenario describes.

Why is "The cells were stained with safranin before crystal violet" not the answer?

The staining sequence has crystal violet first; reversing it is not what happened here.

Why is "Staphylococci lack peptidoglycan and cannot hold crystal violet" not the answer?

Staphylococci have abundant peptidoglycan in the wall.

What concept does this question test?

It tests General microbiology: staining in Microbiology. Prolonged exposure dissolves the crystal violet-iodine complex even from Gram-positive walls, and the counterstain safranin then colours them pink. Gram-positive bacteria have a thick peptidoglycan layer, which normally traps the complex.

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: staining?

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.