ETest

The Etest, also known as the epsilometer test, is a gradient diffusion method used to determine the minimum inhibitory concentration (MIC) of antimicrobial agents against microorganisms. It uses a plastic strip impregnated with a continuous gradient of the antibiotic, enabling the MIC to be read directly from the strip.

Procedure: A standardized inoculum is spread on an appropriate agar to create a bacterial lawn. The Etest strip is placed on the surface and the plate is incubated under suitable conditions. After incubation, an ellipse of inhibition forms around the strip. The MIC is read at the point where the edge of visible growth intersects the scale on the strip, reported in micrograms per milliliter (µg/mL).

Applications and interpretation: Etest provides quantitative MIC values and is used in clinical microbiology to guide antimicrobial therapy for a range of bacteria and some fungi. It is commonly applied to organisms such as Staphylococcus aureus, Enterobacterales, and Pseudomonas aeruginosa. Results are interpreted according to established breakpoints from guidelines issued by CLSI or EUCAST.

Advantages and limitations: The method is relatively simple and requires standard culture facilities. It provides a direct MIC and can test many antibiotics on a single organism. However, results may be affected by diffusion characteristics of certain antibiotics, agar medium, inoculum density, and incubation conditions. It can be more expensive per test than some alternatives and may be unsuitable for antibiotics with poor diffusion or for certain fastidious organisms.