Comparison of Different Phenotypic Approaches to Screen and Detect mecC-Harboring Methicillin-Resistant Staphylococcus aureus

Similar to mecA, mecC confers resistance against beta-lactams, leading to the phenotype of a methicillin-resistant Staphylococcus aureus (MRSA). However, mecC-harboring MRSA pose special difficulties in their detection. The aim of this study was to assess and compare different phenotypic systems for screening, identification, and susceptibility testing of mecC-positive MRSA isolates. A well-characterized collection of mecC-positive S. aureus isolates (n = 111) was used for evaluation. Routinely used approaches were studied to determine their suitability to correctly identify mecC-harboring MRSA including three (semi-)automated antimicrobial susceptibility testing (AST) systems and five selective chromogenic agar plates. Additionally, a cefoxitin disk diffusion test and an oxacillin broth microdilution assay were examined. All mecC-harboring MRSA isolates were able to grow on all chromogenic MRSA screening plates tested. Detection of these isolates in AST systems based on cefoxitin and/or oxacillin testing yielded overall positive agreement with the mecC genotype of 97.3 % (MicroScan WalkAway™, Siemens), 91.9 % (Vitek 2®, bioMérieux), and 64.9 % (Phoenix™, BD). The phenotypic resistance pattern most frequently observed by AST devices was "cefoxitin resistance/oxacillin susceptibility", ranging from 54.1 % (Phoenix) over 83.8 % (Vitek 2) to 92.8 % (WalkAway). The cefoxitin disk diffusion and oxacillin broth microdilution assays categorized 100 % and 61.3 % of isolates to be MRSA, respectively. The chromogenic media tested confirmed their suitability to reliably screen for mecC-harboring MRSA. The AST systems showed false-negative results with varying numbers, misidentifying mecC MRSA as methicillin susceptible S. aureus This study underlines cefoxitin's status as the superior surrogate mecC MRSA marker.