IDENTIFICATION OF MECA-RELATED OXACILLIN RESISTANCE IN STAPHYLOCOCCI BY THE E-TEST AND THE BROTH MICRODILUTION METHOD

A set of 165 strains of different staphylococcal species, 67 Staphyloc occus aureus, 71 novobiocin-sensitive coagulase-negative staphylococci (CNS) and 27 novobiocin-resistant CNS was used. The oxacillin and met hicillin MICs were recorded after 24 and 42 h of incubation at 35 degr ees C and at 30 degrees C. Significantly higher MICs were recorded at 30 degrees C compared with 35 degrees C. While a poor discrimination b etween meed-positive and mecA-negative strains was obtained with methi cillin, the oxacillin MICs enabled identification of resistant strains under certain conditions. The distribution of MICs differed between t he three groups of species. Separation of uninduced meed-positive (gre ater than or equal to 4.0 mg oxacillin/L) and mecA-negative (less than or equal to 2.0 mg oxacillin/L) strains of S. aureus was only achieve d with the E test and after 42 h of incubation. Oxacillin-induction yi elded higher MICs for meed-positive strains of S. aureus, and a separa tion from mecA-negative strains was achieved with the E test after 24 h and with the broth microdilution method after 42 h. Separation of me cA-positive and mecA-negative strains of novobiocin-sensitive CNS requ ired agar supplemented with 5% blood, incubation of MIC trays and E te st for 42 h, and species-specific oxacillin MIC breakpoints (S less th an or equal to 0.5 mg/L and R greater than or equal to 1.0 mg/L). The meed-positive and mecA-negative strains of novobiocin-resistant CNS we re clearly separated after 24 h of incubation by either method.