A comparison of PCR detection of mecA with two standard methods of oxacillin disk susceptibility testing for coagulase-negative staphylococci

Coagulase-negative staphylococci (CNS) are common isolates from blood cultures, and an increasing proportion is now methicillin resistant. The National Committee for Clinical Laboratory Standards (NCCLS) recently issued new criteria for zone sizes applicable to oxacillin disk sensitivity testing for CNS and the British Society of Antimicrobial Chemotherapy (BSAC) has also issued guidelines. This study evaluated two standard methods for oxacillin disk sensitivity testing of 67 CNS isolates from blood cultures, and compared these results with detection of the mecA gene by PCR. Over 94% of mecA-positive isolates were detected by conventional disk testing, with no significant differences between the two methods. In this study, the clinical utility of mecA detection in CNS for the determination of methicillin resistance appears to be limited.