The Etest, also known as the Epsilometer test, is a quantitative microbiological method designed to determine the minimum inhibitory concentration (MIC) of antibiotics and antifungals against bacterial and fungal isolates.[1][2] It utilizes a plastic strip impregnated with a predefined exponential gradient of the antimicrobial agent, spanning typically 15 twofold dilutions from 0.002 to 32 μg/mL, which is applied to an agar plate uniformly inoculated with the test microorganism.[1][3] Following incubation, an elliptical zone of inhibition forms around the strip, and the MIC is determined as the lowest concentration at which bacterial growth is inhibited, marked by the intersection of the ellipse's edge with the strip's scale.[2][3]Developed in the late 1980s and introduced commercially in the early 1990s by bioMérieux (formerly AB Biodisk), the Etest represents an innovative blend of agar diffusion and dilution techniques, providing a cost-effective alternative to traditional broth microdilution methods for antimicrobial susceptibility testing (AST).[4][3] The procedure involves preparing a standardized inoculum (equivalent to a 0.5 McFarland standard), swabbing it onto a suitable agar medium such as Mueller-Hinton agar, placing the strip perpendicular to any potential growth streaks, and incubating at 35–37°C for 18–24 hours (or longer for fastidious organisms).[1][3] Results are interpreted according to guidelines from bodies like the Clinical and Laboratory Standards Institute (CLSI), categorizing isolates as susceptible, intermediate, or resistant based on the MIC value, which aids in guiding targeted antibiotic therapy.[3][2]The Etest is widely applied in clinical microbiology laboratories for diagnosing infections such as sepsis, endocarditis, and those in immunocompromised patients or chronic conditions like cystic fibrosis, where detecting emerging resistance patterns is critical.[1] It supports over 90 antimicrobial agents, including combinations like ceftolozane/tazobactam, and complements automated systems like VITEK 2 for integrated identification and susceptibility testing.[1][5] Advantages include its simplicity, requiring minimal training and equipment, high reproducibility (with agreement rates of 90% or more within ±1–2 dilutions compared to reference methods), and utility in detecting subtle resistance phenotypes like heteroresistant vancomycin-intermediate Staphylococcus aureus (hVISA).[2][4] However, limitations exist, such as potential interpretive errors (up to 30% for certain phenotypes like extended-spectrum beta-lactamase production), inapplicability to some organisms like Cryptococcus neoformans, and higher costs for high-volume testing compared to disk diffusion.[2][3] Overall, the Etest remains a cornerstone of AST, contributing to antimicrobial stewardship efforts amid rising global resistance, with over 2.8 million antibiotic-resistant infections reported annually in the United States alone.[1]
Overview
Definition and Purpose
The Etest is a proprietary, gradient-based antimicrobial susceptibility testing method developed by bioMérieux for quantitatively determining the minimum inhibitory concentration (MIC) of antibiotics or antifungals against various microorganisms.[1] This technique employs a plastic strip with a predefined, stable gradient of antimicrobial concentrations to generate precise MIC values, expressed in μg/mL, typically spanning 15 two-fold dilutions such as from 0.002 to 32 μg/mL, depending on the specific strip and agent.[1][6]The primary purpose of the Etest is to identify the lowest concentration of an antimicrobial agent that inhibits visible growth of a target bacterium or fungus, thereby providing essential quantitative data to inform personalized treatment decisions in infectious diseases.[1] By delivering exact MIC results alongside interpretive categories, it supports antimicrobial stewardship efforts, aids in managing critical infections like sepsis or endocarditis, and helps detect emerging resistance patterns.[1] This is particularly valuable in clinical settings where precise dosing is needed to optimize therapy outcomes while minimizing resistance risks.[6]The Etest is applicable to a broad scope of microorganisms, including Gram-positive bacteria such as Staphylococcus aureus and Enterococcus species, Gram-negative bacteria like Pseudomonas aeruginosa and Enterobacterales, fastidious organisms including Haemophilus influenzae and Streptococcus pneumoniae, as well as certain fungi such as Candida species using antifungal strips.[1][6] It accommodates both non-fastidious and fastidious aerobes, anaerobes, and slow-growing pathogens, making it versatile for routine and specialized susceptibility testing across over 90 antimicrobial agents.[1]
Principle of Operation
The Etest, also known as the epsilometer test, operates on the principle of combining dilution and diffusion methodologies to quantify the minimum inhibitory concentration (MIC) of an antimicrobial agent against a bacterial isolate. The core component is a thin, inert, non-porous plastic strip precoated with a predefined exponential gradient of the antimicrobial on one side and a corresponding MIC scale (in μg/mL) on the opposite side. This gradient typically spans several log2 dilutions, such as from 0.016 to 256 μg/mL for many agents, ensuring a continuous range of concentrations. When the strip is applied to an agar surface inoculated with the test microorganism, the antimicrobial diffuses from the strip into the agar underneath it, establishing a stable concentration gradient that decreases exponentially from the strip's surface.[1][7]The antimicrobial diffuses from the strip into the agar underneath it, establishing a stable, exponential concentration gradient that correlates directly with MIC values. This process occurs passively during incubation, typically at 35–37°C for 16–20 hours, allowing the antimicrobial to diffuse radially while the bacterial inoculum grows. The exponential nature of the gradient ensures that the concentration at any point along the strip's length matches the MIC scale, providing a stable and symmetric diffusion front that minimizes variability from factors like agar depth or incubation time. This mechanism enables precise determination of the lowest concentration inhibiting visible growth, distinguishing Etest from qualitative disk diffusion methods.[8][9]Following incubation, bacterial growth inhibition manifests as a symmetric elliptical zone centered along the strip, with the ellipse's edges demarcating the transition from growth to no growth. The MIC is read at the point where the ellipse's pointed end intersects the strip's scale, representing the concentration at which growth is completely inhibited. This intersection provides a quantitative endpoint, with the elliptical shape arising from the isotropic diffusion in the agar plane perpendicular to the strip. The method's design ensures that the inhibition zone's symmetry facilitates accurate reading, typically to the nearest log2 dilution.[1][7]Etest results are standardized to align with Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) guidelines for MIC interpretation, achieving essential agreement rates of ≥90% with reference broth dilution methods across laboratories. This reproducibility stems from the predefined gradient and controlled diffusion, validated through multicenter studies demonstrating consistency for over 90 antimicrobial agents against various pathogens. Adherence to these guidelines ensures that MIC values can be categorized as susceptible, intermediate, or resistant using established breakpoints.[10]
Procedure
Inoculum Preparation and Inoculation
The inoculum preparation for the Etest involves selecting 3-5 well-isolated colonies from a pure culture on a non-selective agar plate, ideally no more than 18-24 hours old, to ensure active growth and genetic homogeneity. These colonies are emulsified in sterile saline (0.85% NaCl) or Mueller-Hinton broth to create a uniform suspension adjusted to a turbidity of 0.5 McFarland standard, equivalent to approximately 1.5 × 10^8 CFU/mL for most non-fastidious bacteria. This density standardization minimizes variability in microbial growth and minimum inhibitory concentration (MIC) readings, aligning with established antimicrobial susceptibility testing protocols.[11][12]Inoculation proceeds by dipping a sterile, non-absorbent swab into the prepared suspension, rotating it against the inner wall of the tube 5-10 times to remove excess fluid, and then streaking the inoculum across the entire surface of a dried agar plate. The plate is rotated by approximately 60 degrees after each of three successive swabbings to promote even distribution and formation of a uniform microbial lawn, with the final swab run around the plate's periphery to eliminate excess moisture. The inoculated surface is allowed to dry for 3-15 minutes at ambient temperature (20-25°C) in a laminar flow hood, ensuring no visible droplets remain before proceeding. This method achieves a confluent growth essential for accurate gradient diffusion.[12][11]Adjustments for specific organisms enhance reliability. Fastidious bacteria, such as streptococci or Haemophilus influenzae, necessitate suspensions in cation-adjusted Mueller-Hinton broth and inoculation onto supplemented media to support viability during handling. Anaerobes require preparation and inoculation within an anaerobic environment (e.g., glove box or chamber with <1% oxygen) to preserve metabolic integrity, often using prereduced media. Exceptions include mucoid strains of Enterobacteriaceae, adjusted to 1.0 McFarland, or vancomycin-intermediate Staphylococcus aureus (VISA), using a 2.0 McFarland inoculum in brain-heart infusion broth for enhanced detection sensitivity.[11]Quality assurance during inoculum preparation includes immediate verification of turbidity via direct visual comparison to a commercial 0.5 McFarland nephelometer standard or quantitative measurement with a spectrophotometer at 625 nm (optical density 0.08-0.13). Under-inoculation risks faint growth and MIC underestimation, while over-inoculation can cause trailing endpoints; thus, suspensions should be used within 15 minutes of preparation. Reference strains, such as Staphylococcus aureus ATCC 29213, are routinely tested to validate the process yields MIC values within published ranges.[12][11]
Strip Application and Incubation
Strip selection for the Etest begins with choosing the appropriate antimicrobial gradient strips based on the target organism and suspected resistance patterns. For instance, vancomycin strips are commonly selected for testing Gram-positive bacteria such as staphylococci to detect vancomycin-intermediate or -resistant strains. Strips must be handled exclusively with sterile forceps to prevent contamination of the antibiotic gradient or the inoculated agar surface.[11]The application technique involves placing the strips on the agar plate immediately after inoculation, ensuring the predefined concentration gradient is positioned correctly. Up to six strips can be applied per 150 mm plate, arranged perpendicular to one another to minimize overlap of diffusion zones and allow for clear ellipse formation; for smaller 90 mm plates, typically one to two strips are used. Strips should be centered on the inoculated surface with the MIC scale facing upward, and the edges gently pressed down using forceps to achieve full contact with the agar without disturbing the gradient or creating bubbles. This step must be completed within 15 minutes of inoculation to avoid premature diffusion of the antibiotic.[13][14]Incubation follows standard guidelines to promote uniform growth and antibiotic diffusion. For most aerobic bacteria, plates are incubated aerobically at 35–37°C for 18–24 hours in ambient air using non-CO₂ incubators to maintain physiological conditions without altering pH via CO₂ supplementation. Anaerobic bacteria require incubation at 35–37°C for 48 hours in an anaerobic environment, while fungal testing, such as for yeasts on RPMI agar, involves 24–48 hours at 35°C under ambient air conditions. These parameters align with CLSI M100 and EUCAST recommendations, which specify 35 ± 2°C for CLSI and 35 ± 1°C for EUCAST to ensure reproducibility. To prevent agar drying, plates are stacked no higher than five, inverted during incubation to avoid condensation on the lid, and placed in humidified environments if necessary.[15][16][11]
Result Interpretation
After incubation, the Etest plate is examined for bacterial growth inhibition around the strip, forming an elliptical zone due to the concentration gradient. The minimum inhibitory concentration (MIC) is determined by observing the point where the edge of this inhibition ellipse intersects the scale on the strip, selecting the lowest concentration value at that intersection. For zones with hazy or trailing growth, the MIC is read at the sharpest, most distinct edge of the inhibition to ensure accuracy.[13][11]Special cases require careful evaluation to avoid misinterpretation. The presence of microcolonies or isolated colonies within the inhibition zone may indicate heterogeneous resistance, such as in vancomycin-intermediate Staphylococcus aureus (VISA), and warrants further confirmation using methods like population analysis profiling. If no inhibition zone forms around the strip, the MIC is reported as greater than the highest concentration on the scale, suggesting high-level resistance. Conversely, if bacterial growth extends up to or beyond the strip's edge without an observable ellipse, the MIC is less than the lowest concentration on the scale.[13][11]The MIC value is recorded in micrograms per milliliter (μg/mL) as the exact reading from the strip, which may include intermediate values between standard two-fold dilutions. For clinical reporting, this MIC is interpreted against organism-specific and antimicrobial-specific breakpoints established by the Clinical and Laboratory Standards Institute (CLSI) or the European Committee on Antimicrobial Susceptibility Testing (EUCAST) to categorize the isolate as susceptible (S), susceptible with increased exposure (I), or resistant (R). These breakpoints guide therapeutic decisions, with examples including vancomycin MIC ≥16 μg/mL indicating vancomycin-resistant S. aureus (VRSA) per CLSI guidelines.[13]Potential errors in result interpretation arise from technical issues during setup, such as uneven inoculum distribution leading to irregular or jagged inhibition ellipses, which invalidates the test and requires repetition. Plates showing no growth or confluent overgrowth across the agar are also discarded, as they prevent reliable MIC assessment; in such cases, the test must be repeated with verified inoculum density and incubation conditions.[13][11]
Materials and Equipment
Agar Media and Supplements
The Etest, a gradient diffusion method for determining minimum inhibitory concentrations (MICs), relies on specific agar media to support microbial growth and ensure accurate antimicrobial diffusion. For non-fastidious aerobic bacteria, such as Enterobacteriaceae, staphylococci, enterococci, Pseudomonas aeruginosa, and Acinetobacter species, the standard medium is cation-adjusted Mueller-Hinton agar (MHA), which provides a neutral base for consistent antibiotic gradient formation and MIC readability.[11][17]For fastidious aerobes, supplements are added to MHA to meet nutritional requirements. Streptococci and pneumococci require MHA supplemented with 5% sheep blood (CLSI) or 5% defibrinated horse blood plus 20 mg/L β-NAD (EUCAST, designated MH-F) to promote growth and facilitate testing.[17] Haemophilus influenzae uses Haemophilus test medium (HTM) or MH-F agar, while Neisseria gonorrhoeae and meningitidis are tested on chocolate agar (heated blood agar) to supply essential factors like hemin and NAD.[11] For Pseudomonas aeruginosa, cation adjustment in MHA ensures adequate calcium (20-40 mg/L) and magnesium (10-12.5 mg/L) levels, preventing underestimation of MICs for certain antibiotics.[17]Anaerobic bacteria, including Bacteroides species, necessitate enriched media for optimal growth under oxygen-free conditions. The recommended medium is Brucella agar supplemented with 5% sheep blood, 5 μg/mL hemin, and 1 μg/mL vitamin K (or menadione), which supports the strict anaerobes' metabolic needs and maintains antibiotic stability during incubation.[11]For yeasts such as Candida species, antifungal Etest uses RPMI 1640 medium buffered with 0.165 M morpholinepropanesulfonic acid (MOPS) and supplemented with 2% glucose, prepared as agar plates to mimic broth microdilution conditions and yield reproducible MICs.General guidelines for all Etest media include maintaining a pH of 7.2-7.4 at room temperature, verified with a surface pH electrode, to optimize antibiotic activity and microbial growth. Agar depth should be 4 ± 0.5 mm to allow proper strip placement and diffusion without excessive moisture or drying. Quality control involves testing reference strains (e.g., Escherichia coli ATCC 25922, Staphylococcus aureus ATCC 29213) on each new batch of medium, ensuring MICs fall within CLSI or EUCAST specified ranges to validate performance.