Kirby-Bauer antibiotic testing

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In Kirby-Bauer testing, white wafers containing antibiotics are placed on a plate of bacteria. Circles of poor bacterial growth surround some wafers, indicating susceptibility to the antibiotic.

Kirby-Bauer antibiotic testing (KB testing or disk diffusion antibiotic sensitivity testing) is a test which uses antibiotic-impregnated wafers to test whether particular bacteria are susceptible to specific antibiotics. A known quantity of bacteria are grown on agar plates in the presence of thin wafers containing relevant antibiotics such as trimethoprim and nitrofurantoin, commonly used to tackle urinary tract infections. If the bacteria are susceptible to the antibiotics, an area of clearing surrounds the wafer where bacteria are not capable of growing (called a zone of inhibition, or the clear zone).

The size of the zone and the rate of antibiotic diffusion are used to estimate the bacteria's sensitivity to that particular antibiotic. In general, larger zones correlate with smaller minimum inhibitory concentration (MIC) of antibiotic for that bacteria. This information can be used to choose appropriate antibiotics to combat a particular infection.