Evaluation of Oxacillin and Cefoxitin Disk Diffusion and MIC Breakpoints Established by the Clinical and Laboratory Standards Institute for Detection of -Mediated Oxacillin Resistance in Staphylococcus schleiferi.

is a beta-hemolytic, coagulase-variable colonizer of small animals that can cause opportunistic infections in humans. In veterinary isolates, the rate of -mediated oxacillin resistance is significant, with reported resistance rates of >39%. The goal of this study was to evaluate oxacillin and cefoxitin disk diffusion (DD) and MIC breakpoints for detection of -mediated oxacillin resistance in 52 human and 38 veterinary isolates of Isolates were tested on multiple brands of commercial media and according to Clinical and Laboratory Standards Institute (CLSI) methods. Zone diameters and MIC values were interpreted using CLSI breakpoints (CLSI, , 2017) for /, coagulase-negative staphylococci (CoNS), and Results were compared to those of PCR. Twenty-nine of 90 (32%) isolates were positive. Oxacillin inhibition zone sizes and MICs interpreted by breakpoints reliably differentiated -positive and -negative isolates, with a categorical agreement (CA) of 100% and no very major errors (VMEs) or major errors (MEs) for all media. For cefoxitin DD results interpreted using / and CoNS breakpoints, CA values were 85% and 75%, respectively, and there were 72% and 64% VMEs, respectively, and 0 MEs. For cefoxitin MICs interpreted using breakpoints, CA was 81%, and there were 60% VMEs and no MEs. Our data demonstrate that oxacillin DD or MIC testing methods using the current breakpoints reliably identify -mediated oxacillin resistance in , while cefoxitin DD and MIC testing methods perform poorly.