What is an E test in microbiology ( Epsilometer test )

The E-test, or Epsilometer test, is a widely used method in microbiology for assessing the antimicrobial susceptibility of bacteria.

It helps determine the minimum inhibitory concentration (MIC) of an antibiotic needed to inhibit bacterial growth. The MIC is crucial for selecting the appropriate antibiotic dosage in treating infections.

Materials Needed E-test

• E-test strip with a gradient of the antibiotic
• Mueller-Hinton agar plate (or appropriate medium for the organism being tested)
• Inoculum of the bacterial strain, typically standardized to a 0.5 McFarland turbidity standard
• Sterile swabs
• Incubator set to 35–37°C

E test procedure in microbiology

• Select the appropriate medium, usually Mueller-Hinton agar, as it supports the growth of a wide variety of bacteria. For fastidious organisms, specialized agars might be needed.
• Prepare the Bacterial Suspension: Adjust the bacterial culture to match a 0.5 McFarland standard, which provides a uniform inoculum. This can be done by diluting an actively growing culture.
• Dip a sterile swab into the bacterial suspension and spread it evenly across the entire surface of the agar plate, ensuring a uniform lawn of bacteria. This is usually done by swabbing in three directions and then allowing the plate to dry briefly.

• Using sterile forceps, carefully place the E-test strip on the surface of the inoculated agar. Press down gently to ensure full contact between the strip and the agar.
• Place the inoculated plate in an incubator at 35–37°C for 18–24 hours. Incubation time may vary depending on the organism and antibiotic.
• After incubation, examine the plate. A zone of inhibition will form around the E-test strip. The MIC is determined by the point where the elliptical inhibition zone intersects the antibiotic concentration gradient on the strip. The MIC value is read directly from the scale printed on the strip.
• Compare the MIC value to established breakpoints (e.g., provided by CLSI or EUCAST) to determine if the bacterium is susceptible, intermediate, or resistant to the antibiotic.

Advantages of the E-Test

• The test provides an exact MIC value, which is more informative than simpler methods that only give a yes/no answer for susceptibility.
• It is simpler than some other MIC determination methods, such as broth dilution, and doesn’t require complex lab equipment.
• It is particularly helpful for testing fastidious organisms as well as anaerobes.