Evaluation of Oxacillin and Cefoxitin Disk Diffusion and Microbroth Dilution Methods for Detecting -Mediated β-Lactam Resistance in Contemporary Staphylococcus epidermidis Isolates.

Methicillin (β-lactam) resistance in is mediated by the gene, with resistance reported to be as high as 90%. The goal of this study was to evaluate oxacillin and cefoxitin disk diffusion (DD) and broth microdilution (BMD) methods for the detection of -mediated β-lactam resistance in 100 human isolates of (48 -positive isolates and 52 negative isolates). Oxacillin DD tests using the Clinical and Laboratory Standards Institute (CLSI) M100-S28 breakpoints for / accurately differentiated -positive and -negative isolates, with categorical agreement (CA) of 100% and no very major errors (VMEs) or major errors (MEs) identified. Likewise, oxacillin BMD and cefoxitin DD tests using the coagulase-negative species (CoNS) breakpoints were highly reliable for detecting -mediated β-lactam resistance in isolates. For cefoxitin DD and BMD results interpreted using / breakpoints, the CA was 97.6% and 96.2%, respectively. There were 4.9% VMEs for cefoxitin DD with 0% MEs, and 3.6% VMEs and 3.9% MEs for cefoxitin BMD. Oxacillin BMD using / breakpoints yielded the highest VMEs at 17.4% and 90% CA. Our findings demonstrate that oxacillin DD tests using the CLSI M100-S28 breakpoints for / and oxacillin BMD and cefoxitin DD tests using the CoNS breakpoints reliably identified -mediated β-lactam resistance in Using PCR as the gold standard, the PBP2a SA culture colony test (Abbott Diagnostics) exhibited 100% sensitivity and specificity whereas 2 false negatives were identified using the PBP2' latex agglutination test kit (Thermo Fisher Scientific) with sensitivity and specificity of 95.8% and 100%, respectively.