Comparison of three phenotypic techniques for detection of methicillin resistance in Staphylococcus spp. reveals a species-dependent performance

Objectives: To evaluate the usefulness of the cefoxitin screen in Vitek(R) 2 Gram-positive panels for recognizing methicillin-resistant strains of staphylococci. Methods: Seven hundred and ninety-nine non-duplicate isolates of Staphylococcus aureus and coagulase-negative strains were included in the study. Methicillin resistance was measured using PCR for the mecA gene, the CLSI cefoxitin disc diffusion method, the Vitek(R) 2 cefoxitin screen and the Vitek(R) 2 oxacillin susceptibility test. Results: Compared with the molecular detection of methicillin resistance the overall sensitivities and specificities of the phenotypic tests for cefoxitin disc diffusion were 94.9% and 97.0%, for Vitek(R) 2 cefoxitin screen were 94.6% and 93.5% and for Vitek(R) 2 oxacillin susceptibility test were 93.8% and 77.9%. The cephamycin tests (cefoxitin disc diffusion and Vitek(R) 2 screen) were not able to identify mecA-positive strains of Staphylococcus simulans. In addition, the performance of the Vitek(R) 2 system was poor against Staphylococcus cohnii subspecies, Staphylococcus hominis hominis and Staphylococcus saprophyticus. Conclusions: Overall, the performance of the Vitek(R) 2 system for differentiating mecA-positive staphylococci was comparable to PCR and the CLSI disc diffusion method; however, performance was species-dependent. Thus, before accepting the results produced by Vitek(R) 2, species identification may be required.