Optimizing Detection of MEC-Mediated Methicillin Resistance in Coagulase Negative Staphylococcus Species

Abstract

Introduction/Objective Parenteral therapy for invasive staphylococcal infections often comprises lipo/glycopeptides for methicillin resistant (MR) organisms, with extended spectrum penicillins reserved for those confirmed as methicillin susceptible (MS). While phenotypic antimicrobial susceptibility testing (AST) using cefoxitin has high accuracy for differentiating MR/MS for S. aureus, the same, simple approach does not reliably distinguish resistance among all coagulase negative Staphylococcus species (CoNS). As such, clinicians may work under an assumption that MS CoNS cannot be reliably predicted by laboratories, leading to lipo/glycopeptide therapeutic use for invasive infections caused by both MR and MS isolates. To better inform laboratory practice, this study aimed to establish the accuracy of multiple phenotypic and genotypic tests for differentiation of MR/MS among commonly recovered CoNS species producing invasive infections. Methods/Case Report A convenience sample of 155 unique invasive CoNS isolates with historic Vitek 2 and/or disc diffusion (DD) AST results (~50% MR) were retrieved from frozen storage and tested by: (1) cefoxitin and oxacillin DD, (2) a lateral flow immunoassay for PBP2a without beta-lactam induction, and (3) a lab-developed PCR for mecA. A commercial real-time PCR for mecA/C was also employed for a subset of isolates. Results (if a Case Study enter NA) A total of 144 isolates belonging to 11 taxa were evaluated by all tests after accounting for missing, non-viable, and incorrectly cataloged isolates. Using PCR mecA/C detection as the reference standard, categorical agreement for MR/MS status was 95.5% (127/133) for original Vitek GP75 results, 98.6% (142/144) for DD interpreted using updated, species-specific CLSI guidelines, and 100% (144/144) for the lateral flow PBP2a assay. Six MR isolates from five different taxa that were repeatedly mecA-negative by lab-developed PCR confirmed as mecA/C-positive by a commercial assay. Conclusion Although all methods evaluated distinguish MR/MS CoNS with sufficient accuracy, definitive determination is best achieved using off-label/lab-developed PCR for mec or lateral flow immunoassay for PBP2a.