In the Kirby-Bauer test, which is a standardized method used to determine the susceptibility of bacteria to antibiotics, the concentration of antibiotics is typically not expressed in molarity (M) but rather in standardized disk concentrations. Antibiotic-impregnated paper disks are used, and each disk contains a specific amount of antibiotic in micrograms (µg).
For example, common disk concentrations include:
- Penicillin: 10 units or 1 µg
- E. coli: 25 µg
- Gentamicin: 10 µg or 30 µg
The results of the test are interpreted based on the diameter of the zone of inhibition around the antibiotic disk, which indicates whether the bacteria are susceptible, intermediate, or resistant to the antibiotic in question. The specific concentrations of antibiotics used in the disks are standardized by organizations such as the Clinical and Laboratory Standards Institute (CLSI).