Epsilometer Test (E-test) for Antibiotic Susceptibility Testing
Antimicrobial susceptibility testing can be performed using a variety of techniques. The E-test, a recently developed technique, is a modification of the disk diffusion and the agar dilution methods mentioned previously. The simplicity, accuracy, and reliability of the E-test make it appropriate and convenient for Food and Drug Administration (FDA) approved commercialization. Creative Biolabs' deep industry expertise coupled with a multi-platform approach delivers tailored antibiotic susceptibility testing (AST) services to specification.
Overview of E-test for AST
E-test is a new quantitative method to detect antibiotic sensitivity. The E-test is a plastic strip with a predetermined antibiotic gradient fixed on one side and MIC interpretation scale printed on the other. It is based on the diffusion of antibiotics from a plastic strip onto an agar medium, thus providing quantitative and repeatable MICs results for the isolates tested. E-test can also be used to study the antibacterial effect when multiple drugs are used simultaneously.
Fig.1 The E-test principle. (Citron, 1991)
E-test Method at Creative Biolabs
Briefly, the surface of the agar plate is swabbed with the adjusted inoculum suspensions (1~2 × 108 CFU/mL) equivalent to a 0.5 McFarland. The back of the plastic strip is then immersed with different concentration gradients of accumulated antibacterial agent, corresponding to about 20 times dilution of the surface of the medium, and inoculated with the test strain. Many E-test strips with different antibacterial agents can be placed on a single flat surface containing a special agar medium and then incubated with the tested microorganism at different temperatures. After overnight incubation, an inhibition ellipse is seen, the E-test MICs (mcg/mL) value are read directly from the upper of the Petri dish at the point of intersection of the growth ellipse and inhibition zone with the extremity of the strip.
Fig.2 E-test for determining the MIC of antibiotics. (Schumacher, 2018)
Advantages and Disadvantages
The E-test has several advantages over the disc diffusion test:
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The concentration gradient of antibiotics is stable for up to 20h. It is suitable for a variety of pathogens, ranging from rapid growing aerobic and anaerobic bacteria to slow-growing fastidious bacteria.
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The test can be used for finding heterogeneous resistance, or a bacterial strain that forms both susceptible and resistant colonies.
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This method is a simple test procedure capable of determining contamination and testing finicky microorganisms.
In general, the E-test is a convenient AST method that provides exact quantitative MICs at a higher cost.
The E-test is easy to perform and read, suitable for all anaerobes, can be used to test single isolates as needed, and offers the laboratory a reliable method for susceptibility testing of anaerobic bacteria. E-test MICs are generally in very good agreement with those obtained by the agar dilution method. For antibiotic sensitivity testing of live biotherapeutic products (LBP), it is recommended to complement molecular methods with culture-based methods. Creative Biolabs' expert scientists and talented technical support team have got you covered all the way. Please contact us for more information.
References
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Citron, D.M.; et al. Evaluation of the E test for susceptibility testing of anaerobic bacteria. Journal of Clinical Microbiology. 1991, 29(10): 2197-2203.
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Schumacher, A.; et al. In vitro antimicrobial susceptibility testing methods: agar dilution to 3D tissue-engineered models. European Journal of Clinical Microbiology & Infectious Diseases. 2018, 37(2): 187-208.
For Research Use Only. Not intended for use in food manufacturing or medical procedures (diagnostics or therapeutics). Do Not Use in Humans.
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