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

Abstract

Methicillin (β-lactam) resistance in Staphylococcus epidermidis is mediated by the mecA 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 mecA-mediated β-lactam resistance in 100 human isolates of S. epidermidis (48 mecA-positive isolates and 52 mecA negative isolates). Oxacillin DD tests using the Clinical and Laboratory Standards Institute (CLSI) M100-S28 breakpoints for S. pseudintermedius/S. schleiferi accurately differentiated mecA-positive and -negative S. epidermidis 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 Staphylococcus species (CoNS) breakpoints were highly reliable for detecting mecA-mediated β-lactam resistance in S. epidermidis isolates. For cefoxitin DD and BMD results interpreted using S. aureus/S. lugdunensis 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 S. aureus/S. lugdunensis 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 S. pseudintermedius/S. schleiferi and oxacillin BMD and cefoxitin DD tests using the CoNS breakpoints reliably identified mecA-mediated β-lactam resistance in S. epidermidis Using mecA 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.

Keywords: MIC; PBP2a; Staphylococcus epidermidis; broth microdilution; cefoxitin; disk diffusion; mecA; methicillin-resistant Staphylococcus epidermidis; oxacillin.

FIG 1

Distribution of cefoxitin and oxacillin growth inhibition zones and MIC values as determined by DD and BMD testing compiled for media from all three manufacturers. Breakpoints are derived from the CLSI M100-S28 document. (A) Cefoxitin DD (BD) (Hardy and Remel media; n = 300). The dotted line represents the breakpoint for S. aureus and S. lugdunensis; the dashed line represents the breakpoint for CoNS except S. lugdunensis, S. pseudintermedius, and S. schleiferi. (B) Cefoxitin BMD (Difco, BD, and Oxoid media; n = 291). The dotted line represents the breakpoint for S. aureus and S. lugdunensis. (C) Oxacillin DD (BD, Hardy and Remel media; n = 297). The dotted line represents the breakpoint for S. pseudintermedius and S. schleiferi. (D) Oxacillin BMD (Difco, BD, and Oxoid media; n = 291). The dotted line represents the breakpoint for S. aureus and S. lugdunensis; the dashed line represents the breakpoint for CoNS except S. lugdunensis, S. pseudintermedius, and S. schleiferi.