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GCSE Biology · key term
A small paper disc containing an antibiotic, placed on an inoculated agar plate to test its effect on bacterial growth.
Used by OCR
Paper 1 (Foundation), Paper 2 (Foundation), Paper 3 (Higher), Paper 4 (Higher)
Revise: Calculating Bacterial AreasIf an antibiotic disc is placed on the agar, the antibiotic diffuses outwards. Where its concentration is sufficient to kill the bacteria or prevent them growing, a circular clear zone develops around the disc.
A clear zone is measured across its widest part through the centre. The antibiotic disc is normally included in the measured diameter of the whole clear zone.
When comparing antibiotic discs, a larger clear-zone area indicates greater inhibition only when relevant conditions—such as the bacterium, antibiotic concentration, disc size and incubation conditions—are controlled.
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