Introduction: Among the multiple mechanisms of acquired drug resistance in coagulase-negative staphylococci (CoNS), methicillin resistance appears to be the most important from a clinical and epidemiological perspective. According to the Clinical and Laboratory Standard Institute (CLSI) guidelines, the detection of mec genes is the gold standard for identification of staphylococcal methicillin resistance. In this study, we tested the efficiency of routine methods in the detection of methicillin-resistant coagulase-negative staphylococci strains (MRCoNS).

Methods: Methicillin resistance was identified using cefoxitin (30 μg) and oxacillin (1 μg) by the disk diffusion method (DDM) on Mueller–Hinton agar (Becton Dickinson, Franklin Lakes, NJ, USA) per CLSI recommendations. Oxacillin MICs (Minimal Inhibitory Concentrations) were determined by the agar dilution method (ADM) according to CLSI recommendations [M07-A10]. To detect the mecA gene, mecA primers were designed. For mecA-negative CoNS strains, the mecC and mecB genes were tested by simplex PCR.

Results: The sensitivity of the disk diffusion method to oxacillin compared to the detection of the mecA gene was 100%, while the serial dilution method for oxacillin had a lower value (87.1%). A lower sensitivity was obtained for the disk diffusion method for cefoxitin (69.7%), while the specificity of the disk diffusion method for cefoxitin was the highest, at 95.4%. A slightly lower specificity was obtained for oxacillin by the serial dilution and disk diffusion methods, at 90.6% and 87.1%, respectively. According to CLSI guidelines, the reference for phenotypic methods was the detection of mec genes by PCR.

Conclusions: Oxacillin-based methods were highly sensitive in detecting mecA-positive CoNS strains. Due to the heterogeneity of MRCoNS, confirmation of methicillin resistance by different methods seems crucial. This can prevent the misidentification of MRCoNS and failed antibiotic therapy.