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

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ObjectivesTo evaluate the usefulness of the cefoxitin screen in Vitek® 2 Gram-positive panels for recognizing methicillin-resistant strains of staphylococci.MethodsSeven 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® 2 cefoxitin screen and the Vitek® 2 oxacillin susceptibility test.ResultsCompared 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® 2 cefoxitin screen were 94.6% and 93.5% and for Vitek® 2 oxacillin susceptibility test were 93.8% and 77.9%. The cephamycin tests (cefoxitin disc diffusion and Vitek® 2 screen) were not able to identify mecA-positive strains of Staphylococcus simulans. In addition, the performance of the Vitek® 2 system was poor against Staphylococcus cohnii subspecies, Staphylococcus hominis hominis and Staphylococcus saprophyticus.ConclusionsOverall, the performance of the Vitek® 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® 2, species identification may be required.

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