Antibiotic Susceptibility
Antibiotic susceptibility is often measured using the disk diffusion method. Antibiotic-impregnated paper disks are placed on a plate that has been inoculated to form a bacterial lawn. The plates are incubated to allow growth of the bacteria and time for the antibiotics to diffuse into the agar. As a drug diffuses into the agar, its strength decreases.
If an organism is susceptible to an antibiotic, a zone of clearing will appear around the disk where the growth has been inhibited. The size of this zone of inhibition depends on the sensitivity of the bacteria to the specific antibiotic and the antibiotic's ability to diffuse through the agar. After incubation, the clear zones are measured to determine the zone of inhibition for each antibiotic.