Evaluation of a new 3-h hybridization method for detecting the mecA gene in Staphylococcus aureus and comparison with existing genotypic and phenotypic susceptibility testing methods.

A new 3-h hybridization assay for detection of the staphylococcal mecA gene and the Staphylococcus aureus nuclease gene was evaluated by comparing the assay with existing genotypic and phenotypic methods. A total of 275 S. aureus strains were tested, including 257 epidemiologically unrelated strains (135 mecA-positive and 122 mecA-negative; collection I), and 18 strains with known borderline resistance to methicillin (collection II). Complete agreement was obtained for both collections when comparing the new assay with genotypic methods. We further evaluated a range of phenotypic susceptibility methods recommended in Europe and/or USA using the presence of the mecA gene as the defining standard. For collection I a high degree of agreement was found for both Etests (256 strains) and the oxacillin screen plate test (255 strains); the degree of agreement was lower for agar dilution methicillin (250 strains) and oxacillin 1 microg discs (239 strains). For the borderline strains a high degree of agreement was only obtained by the oxacillin screen plate test (17 of 18 strains). The other tests were less accurate, in the following order: agar dilution methicillin, Etest methicillin, Etest oxacillin and oxacillin discs with disagreement for four, five, nine and 13 strains, respectively. In conclusion, the new hybridization assay is a rapid and exact method for detecting the mecA gene and the S. aureus nuclease gene. This study confirms that phenotypic tests for methicillin resistance in S. aureus strains creates both false-susceptible and false-resistant results, especially for borderline resistant strains.