Another name for the disk diffusion test is the Kirby-Bauer test.
Swabs containing a certain organism are uniformly swiped across a medium, known as Mueller-Hinton agar, that has been suspended in a petri dish. Small disks containing fixed amounts of different antibiotics are placed on top of the suspension. Once the plates have been allowed to dry for five minutes, they are then incubated overnight.
As the organisms grow in the agar, circles known as "zones of inhibition" may begin to appear around some of the disks. This occurs when levels of that antibiotic are high enough to prevent the organism's growth.
Organisms are rated as susceptible (weak to), intermediate or resistant to antibiotics depending on the size of the zone of inhibition around each disk.
The amount of agar and organism used, the antibiotic placed on each disk, and the temperature of incubation must be carefully monitored to ensure that results are reliable.
The disk diffusion test is an accurate and inexpensive test commonly used in clinical laboratories to determine the susceptibility of organisms to specific antibiotics. It is useful in helping physicians prescribe drugs that will treat diseases.