|Description:||This 1972 image depicted two Mueller-Hinton agar culture plates that had been used in an antibiotic susceptibility test (AST). The pH of the left plate was labeled as 7.2, while the plate at right was labeled at a value of 6.5, which was slightly more acidic than the left plate. Known as the Kirby-Bauer method, each of the small labeled discs, or wafers, contained an antibiotic cocktail. The light halos surrounding each disc, also known as “reaction zones”, represented regions in which the bacteria on the agar’s surface did not thrive, due to their sensitivity to the antibiotic that had been soaked into these respective discs. Note the subtle differences in bacterial growth, due to the differing pH values, evidenced by the slightly arger reaction zones in the more alkaline plate on the left, indicating reduced bacterial growth around each respective disc.|
These tests which measure the presence or absence of a reaction zone, also take into account the pH of the medium, the concentration of the inoculum, i.e., bacteria, and a number of other factors. Using this method, the sensitivity of a bacterial strain to a given antibiotic, or combination of antibiotics, i.e., antimicrobials, may be determined.