Epsilometer test (E test)- Principle, Procedure, Results, Advantages


Antimicrobial resistance is a major public health concern that threatens the effectiveness of existing treatments for various infections. One of the key steps in managing antimicrobial resistance is to determine the susceptibility of microorganisms to different antibiotics. This can help clinicians to choose the most appropriate therapy and prevent the spread of resistant strains.

There are various methods for testing antimicrobial susceptibility, such as disk diffusion, broth dilution, agar dilution, and automated systems. However, these methods have some limitations, such as being time-consuming, labor-intensive, requiring specialized equipment or media, or having low accuracy or reproducibility.

The Epsilometer test (E-test) is a novel method that overcomes some of these limitations and provides a direct quantification of antimicrobial susceptibility of microorganisms. It is defined as the “exponential gradient” method that combines both the dilution and diffusion principles of antibiotic testing .

The E-test uses rectangular plastic strips that have a predefined, continuous and exponential gradient of antibiotic concentration along one side. The other side of the strip has a numeric scale that indicates the drug concentration in micrograms per milliliter (µg/ml). The strip is applied to an inoculated agar plate and incubated for a specified time. The antibiotic diffuses from the strip into the agar and creates an inhibition zone around it. The minimum inhibitory concentration (MIC) is the lowest concentration of the antibiotic that inhibits the visible growth of the microorganism. The MIC can be read from the scale where the edge of the inhibition zone intersects the strip .

The E-test is a cost-effective, simple and reliable method that can be used for a wide range of antibiotics and microorganisms. It can also detect low levels of resistance or specific phenotypes of resistance that may be missed by other methods. The E-test has been widely used in clinical microbiology laboratories, research settings and epidemiological studies .