General microbiology and sterilisation: role of iodine in Gram stain: 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: role of iodine in Gram stain

Sample item

In the Gram staining sequence of crystal violet, Gram's iodine, alcohol and safranin, what is the specific function of the iodine step?

AIt counterstains Gram-negative bacteria pinkBIt kills the organisms before microscopyCIt removes crystal violet from Gram-negative cell wallsDIt forms a crystal violet-iodine complex that is retained by thick peptidoglycan

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

The answer and the reasoning

D. It forms a crystal violet-iodine complex that is retained by thick peptidoglycan

Gram's iodine acts as a mordant: it combines with crystal violet to form a large insoluble complex inside the cell. Thick peptidoglycan of Gram-positive bacteria traps this complex during alcohol decolourisation. Gram-negative cells lose it and then take up the counterstain.

Option by option

A. It counterstains Gram-negative bacteria pink
The pink colour is due to safranin, the counterstain.
B. It kills the organisms before microscopy
Fixation by heat or methanol, not iodine, kills and fixes the smear.
C. It removes crystal violet from Gram-negative cell walls
Removal of the primary stain is the job of the decolouriser, not iodine.
D. It forms a crystal violet-iodine complex that is retained by thick peptidoglycan (correct)
Correct. Iodine is a mordant that locks crystal violet into the cell as an insoluble complex.

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 D, It forms a crystal violet-iodine complex that is retained by thick peptidoglycan. Gram's iodine acts as a mordant: it combines with crystal violet to form a large insoluble complex inside the cell.

Why is "It counterstains Gram-negative bacteria pink" not the answer?

The pink colour is due to safranin, the counterstain.

Why is "It kills the organisms before microscopy" not the answer?

Fixation by heat or methanol, not iodine, kills and fixes the smear.

Why is "It removes crystal violet from Gram-negative cell walls" not the answer?

Removal of the primary stain is the job of the decolouriser, not iodine.

What concept does this question test?

It tests General microbiology and sterilisation: role of iodine in Gram stain in Microbiology. Thick peptidoglycan of Gram-positive bacteria traps this complex during alcohol decolourisation. Gram-negative cells lose it and then take up the counterstain.

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: role of iodine in Gram stain?

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.