Correlation of oxacillin MIC with mecA gene carriage in coagulase-negative staphylococci

The National Committee for Clinical Laboratory Standards has recently changed the oxacillin breakpoint from greater than or equal to 4 mg/liter to greater than or equal to 0.5 mg/liter to detect methicillin-resistant coagulase-negative staphylococci (CoNS) because the previous breakpoint lacked sensitivity To determine the correlation between the new oxacillin breakpoint and the presence of the mecA gene, 493 CoNS of II species were tested. The presence of the mecA gene was determined qv PCR, and oxacillin susceptibility was determined by the agar dilution method with Mueller-Hinton agar containing 2% NaCl and oxacillin (0.125 to 4.0 mg/liter). The new breakpoint correctly classified all CoNS strains with mecA as methicillin resistant and strains of Staphylcoccus epidermidis, S. haemolyticus, and S. hominis without 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. saprophyticus, 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 identifies strains of CoNS with mecA but is not specific for strains of certain species of CoNS without mecA.