Multi-center and multi-method evaluation of in vitro activities of ceftaroline against S. aureus

This five-site study was performed to assess the reproducibility of ceftaroline MIC and disk results for Staphylococcus aureus. Three commercial broth microdilution, three gradient diffusion and ceftaroline 5μg disk diffusion methods were compared to a reference broth microdilution method against challenge isolates (n = 41) and isolates collected at four European sites (n = 30/site). For four MIC methods (Sensititre and three gradient diffusion methods), 99.0% of consolidated MIC results were within +/- 1 dilution of the reference MIC. Categorical agreement rates based on EUCAST breakpoints for the challenge isolates were 75.6-100% and for disk testing were 78.0-92.7%. There was no clear distinction between isolates with MIC results of 1 and 2mg/L with regard to variation in MIC or molecular genotyping results. The addition of an intermediate category for isolates with MIC results of 2mg/L would help to identify these isolates as borderline susceptible/non-susceptible isolates.