Agar diffusion test

Agar diffusion testing is a method of testing a chemical agents anti-microbial effectiveness by measuring and determining the agents zone of inhibition. In the agar diffusion method, one species of bacteria is uniformly swabbed onto a nutrient agar plate. Chemicals are placed on paper disks. These discs are added to the surface of the agar. During incubation, the chemical diffuses from the disk containing the agent into the surrounding agar. An effective agent will inhibit bacterial growth, and measurements can be made to quantify the size of the zones of inhibition around the disks. The relative effectiveness of a compound is determined by comparing the diameter of the zone of inhibition with values in a standard table.

The agar diffusion test is not used to determine whether a chemical is bactericidal (kills bacteria) or bacteriostatic (inhibits bacteria) - instead this characteristic is determined by the dilution method. In this method the bacterium of interest is placed in a tube containing the chemical which is being tested. The bacterium is then added (subcultured) onto a nutrient agar plate. If the bacterium grows on the nutrient agar the chemical is bacteriostatic; if not, it was killed by the chemical which is then termed "bactericidal."