Evaluation of oxacillin and cefoxitin disk diffusion and microbroth dilution methods for detecting meca-mediated-lactam resistance in contemporary staphylococcus epidermidis isolates

Staphylococcus Ad Hoc Working Group of the CLSI Antimicrobial Susceptibility Testing Subcommittee

Research output: Contribution to journalArticle

2 Scopus citations

Abstract

Methicillin (-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-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 -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-mediate -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.

Original languageEnglish
Article numbere00961-19
JournalJournal of clinical microbiology
Volume57
Issue number12
DOIs
StatePublished - 2019

Keywords

  • Broth microdilution
  • Cefoxitin
  • Disk diffusion
  • MIC
  • MecA
  • Methicillin-resistant staphylococcus epidermidis
  • Oxacillin
  • PBP2a
  • Staphylococcus epidermidis

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