Antimicrobial susceptibility tests measure the ability of an antibiotic or other antimicrobial agent to inhibit bacterial growth in vitro. This ability may be estimated by either the dilution method or the diffusion method.
The dilution test
For quantitative estimates of antibiotic activity, dilutions of the antibiotic may be incorporated into broth or agar medium, which is then inoculated with the test organism. The lowest concentration that prevents growth after overnight incubation is known as the minimum inhibitory concentration (MIC) of the agent. This MIC value is then compared with known concentrations of the drug obtainable in the serum and in other body fluids to assess the likely clinical response.
The diffusion test
Paper discs, impregnated with the antibiotic, are placed on agar medium uniformly seeded with the test organism. A concentration gradient of the antibiotic forms by diffusion from the disc and the growth of the test organism is inhibited at a distance from the disc that is related, among other factors, to the susceptibility of the organism.
There is an approximately linear relation between log MIC, as measured by a dilution test, and the inhibition zone diameter in the diffusion test. A regression line expressing this relation can be obtained by testing a large number of strains by the two methods in parallel (see Fig. 7 and 8).
Fig. 7. Graphic representation of the relationship between log2MIC and the inhibition zone diameter obtained by the diffusion test using discs containing a single concentration of antibiotic
Fig. 8. Interpretation of zone sizes as susceptible, intermediate, and resistant by their relationship to the MIC