Choice of a routine method for detecting methicillin-resistance in staphylococci

Four methods were compared for their abilities to detect methicillin-resistance of Staphylococcus strains using mecA gene PCR analysis in 6 h as the gold standard. 57 Staphylococcus aureus and 100 coagulase negative staphylococci (CNS) were evaluated by the oxacillin disc diffusion method (Kirby-Bauer), the automated API (ATB-plus) system (bioMérieux, La Balme les Grottes, France) in 24 h, the rapid BBL Crystal MRSA ID system (Becton Dickinson, Cockeysville, Md.), the oxillin MICs using the NCCLS agar dilution method in 24 h, and the mecA gene PCR analysis. For S. aureus, the correlation was excellent between the BBL Crystal MRSA ID system and mecA gene PCR analysis (positive predictive value = 100%; negative predictive value = 97%) and oxacillin MIC (positive predictive value = 96%; negative predictive value = 96%). The correlation between BBL Crystal MRSA ID and mecA gene PCR was not reliable for CNS (negative predictive value = 68%). For CNS, the slower routine susceptibility methods to identify intrinsic methicillin-resistance were better: API ATB Staph has a positive predictive value = 94% and a negative predictive value = 82%, and the disc diffusion test has a positive predictive value = 95% and a negative predictive value = 74%. However, BBL Crystal MRSA ID was as reliable as some of the other methods tested for CNS after 6 h incubation when the inoculum was increased: positive predictive value = 94% and a negative predictive value = 77%. These results emphasize that genotypic detection of methicillin-resistance will undoubtedly become important to detect rapidly methicillin-resistance, especially for CNS.