Clinical microbiology: disc diffusion standardisation: 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.

Try it

Microbiology · Clinical microbiology: disc diffusion standardisation

Sample item

In a Kirby-Bauer disc diffusion test, a technician accidentally prepares a bacterial suspension far denser than the 0.5 McFarland standard and lawns it on Mueller-Hinton agar. What is the most likely effect on the result?

ANo change, because zone size depends only on drug potencyBLarger zones of inhibition, which may falsely suggest susceptibilityCSmaller zones of inhibition, which may falsely suggest resistanceDNo growth, because crowded bacteria inhibit each other

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

The answer and the reasoning

C. Smaller zones of inhibition, which may falsely suggest resistance

Disc diffusion results depend on a standard inoculum of about 1.5 x 10^8 CFU per mL (0.5 McFarland), on Mueller-Hinton agar of defined depth and pH. A heavy inoculum means more bacteria must be inhibited and the antibiotic is used up closer to the disc, so zones shrink. A light inoculum does the opposite.

Option by option

A. No change, because zone size depends only on drug potency
Zone size is strongly influenced by inoculum density, agar depth and incubation, not only by the drug.
B. Larger zones of inhibition, which may falsely suggest susceptibility
Larger zones arise from a light inoculum, not a dense one.
C. Smaller zones of inhibition, which may falsely suggest resistance (correct)
A heavy inoculum shrinks the zone and may cause a susceptible isolate to be reported as resistant.
D. No growth, because crowded bacteria inhibit each other
A dense suspension produces confluent growth, not absence of growth.

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 C, Smaller zones of inhibition, which may falsely suggest resistance. Disc diffusion results depend on a standard inoculum of about 1.

Why is "No change, because zone size depends only on drug potency" not the answer?

Zone size is strongly influenced by inoculum density, agar depth and incubation, not only by the drug.

Why is "Larger zones of inhibition, which may falsely suggest susceptibility" not the answer?

Larger zones arise from a light inoculum, not a dense one.

Why is "No growth, because crowded bacteria inhibit each other" not the answer?

A dense suspension produces confluent growth, not absence of growth.

What concept does this question test?

It tests Clinical microbiology: disc diffusion standardisation in Microbiology. 5 x 10^8 CFU per mL (0.5 McFarland), on Mueller-Hinton agar of defined depth and pH. A heavy inoculum means more bacteria must be inhibited and the antibiotic is used up closer to the disc, so zones shrink. A light inoculum does the opposite.

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 Clinical microbiology: disc diffusion standardisation?

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.