BRANCHED-DNA ASSAY FOR DETECTION OF THE MECA GENE IN OXACILLIN-RESISTANT AND OXACILLIN-SENSITIVE STAPHYLOCOCCI

The identification of methicillin-resistant staphylococcus isolates in the clinical laboratory has typically been performed by using methods that detect phenotypic expression of resistance determinants. However , these methods may be difficult to interpret and some isolates do not express resistance until selective pressure is administered. Assays t hat detect genetic determinants are not subject to these limitations a nd have been effective in distinguishing isolates that are capable of expressing the resistance phenotype. In this study, a novel branched D NA (bDNA) hybridization assay was used to test for the mecA gene in 41 6 clinical staphylococcal isolates, The results were compared with tho se obtained by a PCR-based assay and oxacillin disk diffusion. For 155 Staphylococcus aureus and 261 coagulase-negative Staphylococcus isola tes, the bDNA assay and PCR results mere 100% concordant. Among the S. aureus isolates, 20 mere MecA(+) and 135 were MecA(-). For the coagul ase-negative staphylococci, 150 were MecA(+) and 111 were MecA(-). The results from the genotypic detection methods were compared with those obtained by oxacillin disk diffusion. No discrepancies were detected among the S. amarts isolates; however, 10 coagulase-negative isolates were MecA(+) but oxacillin sensitive and 1 isolate was MecA(-) but oxa cillin resistant. Oxacillin resistance was induced in 6 of the 10 MecA (+) isolates previously classified as oxacillin sensitive. These resul ts suggest that the bDNA method described here is a sensitive and effi cient method for detection of methicillin resistance in staphylococci and that genetic detection methods may be useful for detection of pote ntial methicillin resistance in the clinical laboratory.