Oxacillin susceptibility testing of coagulase-negative staphylococci using the disk diffusion method and the Vitek GPS-105 card

One hundred and ninety-three isolates of coagulase-negative staphylococci (CoNS) were tested against oxacillin by agar dilution, disk diffusion, and Vitek (GPS-105 card), and the presence of the mecA gene determined by multiplex PCR. The results obtained by all testing methods were in agreement for 190 isolates. Two mecA-negative isolates ( S. lugdunensis and S. haemolyticus) had MICs of ≤0.25 μg/ml by agar dilution and Vitek but were resistant by disk diffusion. One mecA-positive isolate was resistant by Vitek and disk diffusion but had an agar dilution MIC of ≤0.25 μg/ml. For the species of CoNS tested, oxacillin susceptibility results obtained with the Vitek GPS-105 card and disk diffusion correlated well with results obtained by National Committee for Clinical Laboratory Standards agar dilution and with the presence of the mecA gene.