Why does a clear zone form in the lawn around an antibiotic disc?

The antibiotic diffuses out of the disc into the agar, and close to the disc its concentration is high enough to stop the bacteria growing, so no lawn develops there.

How should the size of a zone of inhibition be measured and recorded?

The diameter is measured in millimetres through the centre of the disc, usually twice at right angles and averaged, and it is often converted into the area of the zone using A = πr².

A zone of inhibition has a diameter of 18 mm. Calculate its area to three significant figures.

The radius is 9.0 mm, so the area is π × 9.0² = 254 mm².

State three variables that must be kept the same when the effects of two antibiotics are compared on one plate.

The volume and concentration of antibiotic soaked into each disc, the depth of the agar in the plate, and the age and density of the bacterial culture used to make the lawn.

Why should a disc soaked only in the solvent be placed on the plate?

It acts as a control, showing that any clear zone seen is caused by the antibiotic itself and not by the liquid it was dissolved in.

OCR A-Level Biology B H422

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Controlling Communicable Diseases

Investigating antibiotics, disinfectants and hygiene

3 things to know
Components 01, 02 and 03

Overview

Recall questions on testing antibiotics with discs on a bacterial lawn, measuring zones of inhibition and calculating their area, controls and control variables, penicillin against Gram-positive and Gram-negative bacteria, bacteriostatic and bactericidal antibiotics, disinfectants and antiseptics, and handwashing with soap.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Process

Distinguish the two classes of antibiotic action and describe how you would find out which one you have.

Process

Explain how a clear zone appears around an antibiotic disc and why its diameter does not rank two antibiotics.

Key term

How is the effect of an antibiotic on a bacterium investigated using agar plates?

Questions from this subtopic

Each one with the model answer underneath.

1
How is the effect of an antibiotic on a bacterium investigated using agar plates?
Model answer
A lawn of the bacterium is spread evenly over sterile agar, paper discs soaked in the antibiotic are placed on the surface with sterile forceps, the plate is taped, inverted and incubated, and the clear zone around each disc is then measured.
2
Why does a clear zone form in the lawn around an antibiotic disc?
Model answer
The antibiotic diffuses out of the disc into the agar, and close to the disc its concentration is high enough to stop the bacteria growing, so no lawn develops there.
3
How should the size of a zone of inhibition be measured and recorded?
Model answer
The diameter is measured in millimetres through the centre of the disc, usually twice at right angles and averaged, and it is often converted into the area of the zone using A = πr².
4
A zone of inhibition has a diameter of 18 mm. Calculate its area to three significant figures.
Model answer
The radius is 9.0 mm, so the area is π × 9.0² = 254 mm².
5
State three variables that must be kept the same when the effects of two antibiotics are compared on one plate.
Model answer
The volume and concentration of antibiotic soaked into each disc, the depth of the agar in the plate, and the age and density of the bacterial culture used to make the lawn.
6
Why should a disc soaked only in the solvent be placed on the plate?
Model answer
It acts as a control, showing that any clear zone seen is caused by the antibiotic itself and not by the liquid it was dissolved in.

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