MIC matters: Enhancing the accuracy of Enterococcus susceptibility testing for vancomycin and linezolid beyond disk diffusion- A cross-sectional study.

BACKGROUND: The escalating resistance of Enterococcus species to vancomycin and linezolid presents a critical therapeutic challenge. Accurate susceptibility testing is essential for guiding treatment; however, the conventional disk diffusion method is limited by inconsistent reproducibility and an inability to quantify Minimum Inhibitory Concentrations (MIC). Therefore, this study aims to evaluate the comparative diagnostic performance of disk diffusion versus the E-test method for assessing vancomycin and linezolid susceptibility in enterococcal isolates.

MATERIALS AND METHODS: In this cross-sectional study at a tertiary care hospital, clinical isolates were processed using standard microbiological methods. Susceptibility to vancomycin and linezolid was screened via disk diffusion; isolates demonstrating intermediate or resistant phenotypes underwent MIC determination using the E-test. Results were interpreted per CLSI-2024 guidelines. Statistical significance was analyzed using Chi-square and Fisher's exact tests.

RESULTS: Among 250 enterococcal isolates, vancomycin resistance was significantly higher by disk diffusion (14.4%, 36/250) compared to the E-test (9.6%, 24/250; p=0.020). Linezolid resistance was identified in 6% (15/250) of isolates by disk diffusion versus 3.2% (8/250) by E-test, with no statistically significant difference observed (p=1).

CONCLUSION: The E-test combines the technical simplicity of diffusion methods with quantitative precision. Given the significant discrepancies observed-particularly for vancomycin-this study warrants the confirmation of all non-susceptible disk diffusion results using MIC methods. While micro-broth dilution remains the gold standard, the E-test serves as a rapid and reliable alternative for timely therapeutic decision-making in clinical settings.