How does an E-test determine the MIC?

How does an E-test determine the MIC?

• A. By using a strip with a linear antibiotic concentration gradient; the MIC is read at the intersection of the inhibition ellipse with the strip
• B. By measuring the zone of inhibition diameter
• C. By color change in a broth microdilution well
• D. By comparing growth to a reference panel

Answer: (A)

An E-test determines MIC by using a plastic strip that contains a continuous gradient of antibiotic concentrations. After the strip is placed on an agar plate inoculated with the test organism, the antibiotic diffuses into the agar and creates a concentration gradient. The bacterial growth is inhibited up to a point along the strip, forming an inhibition ellipse. The MIC is read directly from the scale on the strip at the intersection where growth first ceases—that is, the lowest antibiotic concentration that prevents visible growth. This approach is different from reading a zone diameter in disk diffusion, which uses a circular cleared area to interpret susceptibility. It also isn’t based on color change in broth microdilution or on comparing growth to a reference panel; instead, it provides a precise MIC value directly from the gradient strip.

An E-test determines MIC by using a plastic strip that contains a continuous gradient of antibiotic concentrations. After the strip is placed on an agar plate inoculated with the test organism, the antibiotic diffuses into the agar and creates a concentration gradient. The bacterial growth is inhibited up to a point along the strip, forming an inhibition ellipse. The MIC is read directly from the scale on the strip at the intersection where growth first ceases—that is, the lowest antibiotic concentration that prevents visible growth.

This approach is different from reading a zone diameter in disk diffusion, which uses a circular cleared area to interpret susceptibility. It also isn’t based on color change in broth microdilution or on comparing growth to a reference panel; instead, it provides a precise MIC value directly from the gradient strip.