Evaluation of Oxacillin and Cefoxitin Disk and MIC Breakpoints for Prediction of Methicillin Resistance in Human and Veterinary Isolates of Staphylococcus intermedius Group

Abstract

Staphylococcus pseudintermediusis a coagulase-positive species that colonizes the nares and anal mucosa of healthy dogs and cats. Human infections withS. pseudintermediusrange in severity from bite wounds and rhinosinusitis to endocarditis; historically, these infections were thought to be uncommon, but new laboratory methods suggest that their true incidence is underreported. Oxacillin and cefoxitin disk and MIC tests were evaluated for the detection ofmecA- ormecC-mediated methicillin resistance in 115 human and animal isolates of theStaphylococcus intermediusgroup (SIG), including 111Staphylococcus pseudintermediusand 4Staphylococcus delphiniisolates, 37 of which weremecApositive. The disk and MIC breakpoints evaluated included the Clinical and Laboratory Standards Institute (CLSI) M100-S25Staphylococcus aureus/Staphylococcus lugdunensisoxacillin MIC breakpoints and cefoxitin disk and MIC breakpoints, the CLSI M100-S25 coagulase-negativeStaphylococcus(CoNS) oxacillin MIC breakpoint and cefoxitin disk breakpoint, the CLSI VET01-S2S. pseudintermediusoxacillin MIC and disk breakpoints, and the European Committee on Antimicrobial Susceptibility Testing (EUCAST)S. pseudintermediuscefoxitin disk breakpoint. The oxacillin results interpreted by the VET01-S2 (disk and MIC) and M100-S25 CoNS (MIC) breakpoints agreed with the results ofmecA/mecCPCR for all isolates, with the exception of one false-resistant result (1.3% ofmecA/mecCPCR-negative isolates). In contrast, cefoxitin tests performed poorly, ranging from 3 to 89% false susceptibility (very major errors) and 0 to 48% false resistance (major errors). BD Phoenix, bioMérieux Vitek 2, and Beckman Coulter MicroScan commercial automated susceptibility test panel oxacillin MIC results were also evaluated and demonstrated >95% categorical agreement withmecA/mecCPCR results if interpreted by using the M100-S25 CoNS breakpoint. The Alere penicillin-binding protein 2a test accurately detected allmecA-positive isolates, although for four isolates, cefoxitin induction was required prior to testing. These data demonstrate that the cefoxitin surrogate test does not reliably detect the presence ofmecAinS. pseudintermediusisolates and that laboratories should perform oxacillin disk or MIC tests of these isolates when they are encountered.