Prediction of mecA-positive coagulase-negative staphylococci: assessment of different phenotypic methods, breakpoints, culture media and culture conditions

Sensitivity for the detection of mecA-positive coagulase-negative staphylococci (CNS) was evaluated for different breakpoints of resistance for oxacillin using three different susceptibility tests, either on Mueller-Hinton agar supplemented with 2% NaCl (MH-NaCl agar) or on paper disc method agar supplemented with 5% defibrinated blood (PDM-blood agar). The Etest, multipoint inoculation test and disc diffusion test showed comparable sensitivity (0.96, 0.96 and 0.95, respectively) using an oxacillin breakpoint of > or = 0.5 mg/L or < or = 17 mm for the disc test, after incubation at 35 degrees C for 24 h on MH-NaCl agar. The sensitivity decreased for breakpoints > or = 1 mg/L and when PDM-blood agar was used instead of MH-NaCl agar.