Minimum inhibitory concentrations (MIC) of antibiotics are routinely determined by broth or agar serial dilution methods or by agar diffusion methods. More recently, e-test was developed to reduce the time, labor, and materials used in MIC determination assays.
E-test is based on arraying a concentration gradient of each antibiotic on a polymer strip. Concentration values are marked on the other side of the strip so that one can easily locate corresponding concentrations. E-strips, also known as “epsilometers”, are commercially prepared by microdispersing robotic machines that can deliver nanoliter volumes of antibiotic concentration along the strip.
Each antibiotic strip is laid on the surface of an inoculated agar plate. An elliptical zone of inhibition develops with the broad end at the top of the strip with the highest antibiotic concentration and the narrowest end at the lowest amount of antibiotic that can inhibit bacterial growth, i.e., minimum inhibitory concentration. Several different antibiotic e-strips can be tested simultaneously on the same agar plate. Therefore MIC’s can be determined for many antibiotics in a single step with no need for dilution in broth or agar.
Also, e-test is applied routinely as a “culture sensitivity test” in some medical laboratories in place of the traditional Kirby-Bauer method. In addition to reduction of time and effort, e-test yields sensitivity test results in quantitative terms which makes interpretation of results more precise and easier than routine methods.
This Flash-animated movie illustrates the concept and steps for carrying out the e-test for MIC determination. This original animated artwork movie can be used in microbiology classrooms.