How does E-testing determine MIC?

• A. A gradient strip on agar with a predefined antibiotic gradient that produces a drop-shaped inhibition ellipse
• B. Disk diffusion on agar with fixed concentrations
• C. Broth microdilution in microtiter plates
• D. Genotypic resistance testing

Answer: (A)

E-testing determines the MIC by using a plastic strip that carries a continuous antibiotic concentration gradient placed on an agar plate inoculated with the test organism. As the plate incubates, bacteria grow wherever the antibiotic concentration is not inhibitory, while a clear inhibition zone forms along the strip where the concentration is sufficient to stop growth. The MIC is read at the point where the edge of this inhibition intersects the scale on the strip, which typically appears as a drop-shaped or elliptical zone. This provides a direct, quantitative value for the lowest concentration that prevents visible growth.

This approach versus other methods: with disk diffusion, fixed concentrations on disks create a halo whose size is used to infer susceptibility but does not give a precise MIC. Broth microdilution uses serial dilutions in liquid to determine the MIC, but in a different format. Genotypic resistance testing looks for resistance genes rather than measuring inhibition of growth to a concentration. The gradient strip in E-testing uniquely yields a numeric MIC directly from the zone-intersection reading.