The Epsilometer Test (E-test) for Antimicrobial Susceptibility Testing
The Epsilometer test, commonly known as the E-test, is a sophisticated, yet user-friendly, in vitro method designed to determine the Minimum Inhibitory Concentration (MIC) of various antimicrobial agents against microorganisms. It represents a significant advancement in clinical microbiology, bridging the gap between the qualitative data provided by disk diffusion and the quantitative, yet labor-intensive, results of traditional broth or agar dilution methods. The E-test is a crucial tool in the rational use of antimicrobial agents, as it provides precise, on-scale quantitative MIC data, which is a prerequisite for managing critical infections, such as sepsis, and guiding appropriate therapeutic decisions in complex cases. It is a proprietary system, initially developed by AB BIODISK and now manufactured by bioMérieux, and is applicable to a wide array of drug-bug combinations, including both fastidious and non-fastidious organisms.
Principle of the E-test: Combining Diffusion and Dilution
The fundamental principle of the E-test lies in its unique combination of the antibiotic dilution and diffusion concepts. The core component is a rectangular, non-porous plastic strip, typically 5 mm wide and 60 mm long, which has a predefined, continuous, and stable exponential gradient of antibiotic concentration immobilized on one side. The other side is printed with a numerical scale that indicates the drug concentration in $mu$g/mL. This gradient, covering a continuous range of concentrations, is the key innovation that allows for a quantitative result from a simple agar-based diffusion test.
When the E-test strip is applied to the surface of an agar plate that has been uniformly inoculated with the test organism, the antimicrobial agent immediately begins to diffuse out of the plastic carrier and into the agar medium. This rapid release establishes a stable and continuous concentration gradient of the antibiotic beneath and around the strip. The concentration is highest at the ‘E’ end of the strip and decreases logarithmically toward the other end. This process simulates a series of multiple two-fold dilutions without the need for preparing them manually.
After the plate is incubated under appropriate conditions for microbial growth (e.g., 37°C for 18-24 hours), the bacterial growth becomes visible. A symmetrical, tear-drop or ellipse-shaped zone of growth inhibition forms along the length of the strip. The concentration of the antibiotic is high enough to inhibit growth near the strip’s high-concentration end, but decreases until it reaches a point where it is no longer inhibitory. This intersection point of the inhibition ellipse edge with the E-test strip’s printed scale directly indicates the Minimum Inhibitory Concentration (MIC) value of the antimicrobial agent against the tested microorganism. The simplicity and visual nature of the inhibition ellipse make the reading straightforward.
Procedure: From Inoculum to Incubation
The procedure for performing an E-test requires careful attention to standardization to ensure reproducible results. The first critical step is the preparation of the bacterial inoculum. Several well-isolated colonies of the test strain are selected from an overnight culture plate and emulsified in sterile saline or another appropriate broth. The turbidity of this suspension must then be carefully adjusted to match a 0.5 McFarland standard (approximately 1-2 x 10⁸ CFU/mL) to ensure a standardized, confluent lawn of growth upon incubation. For certain fastidious or mucoid organisms, an adjustment to a 1.0 McFarland standard or use of specialized media may be necessary.
Once the inoculum is prepared, a sterile cotton swab is dipped into the suspension, and excess fluid is removed by rotating the swab against the inside of the tube above the fluid level. The entire surface of an appropriate agar plate—typically Mueller Hinton Agar (MHA) for non-fastidious bacteria, but others like Blood Agar for fastidious ones (e.g., *Streptococcus pneumoniae*)—is then streaked uniformly in three different directions, typically by rotating the plate approximately 60 degrees after each streaking, to create a confluent lawn of growth. The plate must be allowed to dry for a brief period (around 5-15 minutes) before the strips are applied.
The E-test strip, which must be removed from the freezer and kept at room temperature for at least 30 minutes prior, is then carefully placed onto the inoculated agar surface using sterile forceps, ensuring the printed scale faces upwards and the strip is pressed gently but firmly against the agar. One must be careful not to remove or replace a strip once it has touched the agar. Multiple strips (e.g., up to six on a large 150 mm plate) can be placed, provided they do not touch each other or the edge of the plate, which would interfere with the antibiotic diffusion gradients. The plates are then incubated at the appropriate temperature (typically 35°C) and atmosphere (e.g., CO₂ for *Haemophilus* or *Streptococcus pneumoniae*) for the specified duration (usually 18-24 hours).
Reading and Interpretation of Results
Following the prescribed incubation period, the E-test results are read by observing the symmetrical inhibition ellipse formed on the plate. The MIC value is read directly from the printed scale on the strip at the point where the ellipse edge intersects the strip. This intersection point represents the lowest concentration of the antimicrobial agent that completely inhibits visible bacterial growth. Reading is done by viewing the strips from the top of the plate. It is essential to read the MIC at the point of complete inhibition of all growth, ignoring any thin films of growth or isolated colonies within the inhibition zone, particularly at the extreme edges of the strip.
If the ellipse intersects the scale between two printed values—which represents a two-fold dilution—the result should always be rounded up to the next highest concentration value. For example, if the intersection occurs between 2 and 3 $mu$g/mL, the MIC is read as 3 $mu$g/mL. Furthermore, if the intersection points on either side of the strip differ, the higher value should be recorded as the MIC. The final quantitative MIC value ($mu$g/mL) is then interpreted qualitatively as Susceptible (S), Intermediate (I), or Resistant (R) by comparing it to the established clinical breakpoint values recommended by standardized international guidelines, such as those published by the Clinical and Laboratory Standards Institute (CLSI). This interpretation step is critical for guiding the physician on the effective use of the antibiotic.
Key Advantages of the E-test Method
The E-test offers several distinct advantages that have cemented its role in modern clinical and research laboratories. Firstly, it provides a **direct, quantitative MIC value**, which is a significant improvement over qualitative disk diffusion zone diameters. This precise, on-scale quantitative data is crucial for the management of critical infections and for determining the appropriate dosage and choice of antimicrobial agent, particularly in patients with severe infections or for whom a high level of resistance is suspected.
Secondly, the E-test is highly effective and particularly useful for **fastidious, slow-growing, and nutritionally deficient microorganisms** (e.g., anaerobes, *Streptococcus pneumoniae*, *Haemophilus influenzae*, *Neisseria gonorrhoeae*). These organisms often pose challenges to traditional methods, but the E-test provides reliable and easily interpretable results for them on appropriate agar media. It also aids in detecting or confirming **low levels of resistance** or specific, emerging antimicrobial resistance phenotypes like ESBLs, MBLs, and AmpC enzymes, which can be critical for infection control.
Thirdly, it offers tremendous **flexibility and convenience**. A laboratory can select one or more specific antibiotics to test against a single isolate on a single plate, eliminating the need for pre-prepared microdilution panels for all drug-bug combinations. This flexibility makes it cost-effective for testing an unusual drug or for use in smaller laboratories. Additionally, the test is **technically simple to perform and interpret** and requires minimal specialized equipment, leading to high intra- and inter-laboratory reproducibility. The stability of the antimicrobial gradient on the strip also allows for reliable testing over extended incubation periods for slow-growing organisms.
Finally, the E-test can be used to investigate the **synergistic potential of combination therapies** and is one of the most efficient methods for generating on-scale MIC values across 15 doubling dilutions. Its ability to provide accurate and quantitative results for a wide range of drug-bug combinations makes it an indispensable tool for helping to guide patient therapy and supporting broader antimicrobial stewardship efforts.