Correlation of oxacillin MIC with mecA gene carriage in coagulase-negativestaphylococci

The National Committee for Clinical Laboratory Standards has recently chang ed the oxacillin breakpoint from greater than or equal to 4 mg/liter to gre ater than or equal to 0.5 mg/liter to detect methicillin-resistant coagulas e-negative staphylococci (CoNS) because the previous breakpoint lacked sens itivity To determine the correlation between the new oxacillin breakpoint a nd the presence of the mecA gene, 493 CoNS of II species were tested. The p resence of the mecA gene was determined qv PCR, and oxacillin susceptibilit y was determined by the agar dilution method with Mueller-Hinton agar conta ining 2% NaCl and oxacillin (0.125 to 4.0 mg/liter). The new breakpoint cor rectly classified all CoNS strains with mecA as methicillin resistant and s trains of Staphylcoccus epidermidis, S. haemolyticus, and S. hominis withou t mecA as methicillin susceptible. The breakpoint of greater than or equal to 0.5 mg/liter,vas not specific for S, cohnii, S, lugdunensis, S. saprophy ticus, S. warneri, and S. xylosus, in that it categorized 70 of 74 strains of these species without mecA (94.6%) as methicillin resistant. The results of this study indicate that the new oxacillin breakpoint accurately identi fies strains of CoNS with mecA but is not specific for strains of certain s pecies of CoNS without mecA.