Phenotypic detection of mecC-MRSA: Cefoxitin is more reliable than oxacillin

Objectives: To investigate the reliability of cefoxitin and oxacillin for the detection of mecC-positive Staphylococcus aureus. Methods: The susceptibility to cefoxitin and oxacillin of 62 mecC-positive S. aureus isolateswas investigated using broth microdilution, agar dilution, Etest and disc diffusion on different types of media. The data were interpreted for the utility of cefoxitin and oxacillin in conjunction with the stated methodologies for the detection of mecC-positive isolates. Results: Cefoxitin with Mueller-Hinton media fromBecton Dickinson and Oxoid detected all mecC-positive isolates when tested by broth microdilution, agar dilution and disc diffusion. By Etest, one isolate was falsely susceptible. Mueller-Hinton agar from bioMérieux was substantially less able to detect these isolates. One isolate was falsely susceptible by agar dilution when using Iso-Sensitest and Columbia agar. Disc diffusion using cefoxitin on Iso- Sensitest agar missed 29% of the isolates. For oxacillin, only agar dilution on Columbia agar+2% NaCl was able to detect all mecC-positive isolates successfully. Conclusions:Cefoxitin used withEUCASTmethodologyandoxacillin used with agardilutiononColumbia agar+2% NaCl detected all mecC-positive isolates. These methods with their concomitant agars should be preferred over Iso-Sensitest, which is recommended by the BSAC. It should be noted that for disc diffusion Mueller-Hinton media from bioMérieux performed poorly, with 26%-47% of mecC isolates being falsely susceptible.