Reproducible measurement of vancomycin MICs within the susceptible range in Staphylococcus aureus by a broth microdilution method with a “quasi-continuum” gradient of antibiotic concentrations

The availability of reproducible broth microdilution (BMD) methods including inter log₂ antibiotic dilutions for measuring Staphylococcus aureus (SA) vancomycin minimum inhibitory concentrations (MICs) within the susceptible range is needed to elucidate the impact of vancomycin MICs on clinical outcomes of invasive SA infections. Here, we report on the development of a very precise BMD method that incorporates the following incremental antibiotic concentrations: 0.50, 0.62, 0.75, 0.87, 1.0, 1.25, 1.40, 1.50, 1.60, 1.75, and 2.0 μg/mL. The intra- and inter-assay coefficients of variation of this method were around 20%. The mean of the differences in MIC values for all isolates obtained across two independent runs performed at one center was 0.04 μg/mL [95% confidence interval (CI), 0.011–0.07 μg/mL] and that for ten isolates measured at two different centers was 0.04 μg/mL (95% CI, 0–13 μg/mL). Vancomycin MIC values differed by less than 0.1 μg/mL between runs for most isolates. Storage of isolates at −20 °C for up to 3 months had no impact on the vancomycin MIC values. The mean vancomycin MIC values obtained by the Etest using a standard inoculum (0.5 McFarland) were significantly higher (p ≤ 0.001) than those measured by BMD and the MIC values measured by the two methods correlated poorly (Rho, 0.319; p = 0.148). Nevertheless, the mean MIC values measured by the Etest using lower inocula (10⁷ or 10⁶ CFU/mL) and those measured by BMD were comparable and correlated significantly (p = 0.004 for 10⁷ CFU/mL and p = 0.029 for 10⁶ CFU/mL).