Etest: Determining Minimal Inhibitory Concentration (MIC)

What does the Etest determine?

A. susceptibility

B. MBC

C. MIC

D. both susceptibility and MIC

E. both MBC and MIC

Answer:

The Etest determines the Minimal Inhibitory Concentration (MIC).

Explanation:

The Etest is used primarily to ascertain the Minimal Inhibitory Concentration (MIC), which represents the lowest concentration of an antibacterial drug that can inhibit the visible growth of a bacterial strain. Modeled as a combination of Kirby-Bauer disk diffusion test and dilution methods, Etest deploys plastic strips containing a gradient of an antibacterial substance on an agar plate. As the bacterial isolate grows, the antibiotic diffuses from the strip and interacts with the bacterial cells, establishing an elliptical zone of inhibition. At the intersection of this inhibition zone with the gradient on the strip, the MIC is identified.

Etest allows multiple MIC determinations concurrently as strips with different antimicrobials can be placed on the same plate. The method is thus efficient in double-checking susceptibility and comparing potency between antimicrobials. However, the Etest does not determine the Minimal Bactericidal Concentration (MBC), which is the lowest drug concentration that can kill a substantial percentage of the bacterial starting culture. Hence, the Etest validates both Susceptibility and MIC of multiple antimicrobials, but not MBC.