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

Abstract

Staphylococcus schleiferi is a beta-hemolytic, coagulase-variable colonizer of small animals that can cause opportunistic infections in humans. In veterinary isolates, mecA-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 minimum inhibitory concentration (MIC) breakpoints for detection of mecA-mediated oxacillin resistance in 52 human and 38 veterinary isolates of S. schleiferi. Isolates were tested on multiple brands of commercial media following Clinical and Laboratory Standards Institute (CLSI) methods. Zone diameters and MIC values were interpreted using breakpoints in the CLSI M100S 27^th edition for Staphylococcus aureus/Staphylococcus lugdunensis, coagulase-negative staphylococci (CoNS), and Staphylococcus pseudintermedius. Results were compared to mecA PCR. Twenty-nine of 90 (32%) isolates were mecA positive. Oxacillin zone sizes and MICs interpreted by S. pseudintermedius breakpoints reliably differentiated mecA positive and mecA negative isolates, with a categorical agreement (CA) of 100% and no very major errors (VMEs) or major errors (MEs) on all media. For cefoxitin DD interpreted using S. aureus/S. lugdunensis and CoNS breakpoints, CA was 85% and 75%, and there were 72% and 64% VMEs and 0 MEs, respectively. For cefoxitin MICs interpreted using S. aureus/S. lugdunensis 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 S. pseudintermedius breakpoints reliably identify mecA-mediated oxacillin resistance in S. schleiferi, while cefoxitin DD and MIC testing perform poorly.