Evaluation of discrepancies between oxacillin and cefoxitin susceptibility in coagulase-negative staphylococci

The Clinical and Laboratory Standards Institute (CLSI) recommends testing coagulase-negative staphylococci (CoNS) strains to determine resistance against oxacillin by testing for mecA, PBP2a, or with cefoxitin disk. However, discrepant results of resistance to oxacillin and susceptibility to cefoxitin were found. In this study, we aimed to investigate the oxacillin resistance and cefoxitin susceptibility of CoNS in Taiwan. Of 9,017 strains collected from 2005 to 2010, 131 (1.5%) of the isolates were oxacillin-resistant and cefoxitin-susceptible. Species identification was carried out using the Vitek 2 system or 16S ribosomal RNA sequencing. Oxacillin minimum inhibitory concentrations (MICs) were examined by the agar dilution method. The presence of mecA and the activity of β-lactamase were performed by polymerase chain reaction (PCR) and Cefinase disks, respectively. Overall, 33% (43/129) of the strains carried mecA and 43% (37/86) of mecA-negative isolates tested positive for β-lactamase. The remaining 49 isolates were negative for both mecA and β-lactamase, and were mainly Staphylococcus cohnii ssp. urealyticus and S. saprophyticus (oxacillin MICs 0.5-2 μg/ml) obtained from bloodstream and urinary tract infections. Our study suggests that incorrect reporting can be found in CoNS using cefoxitin disk alone to determine the susceptibility to oxacillin, and the strains should be further tested for oxacillin MICs and detection of the mecA gene or β-lactamase activity.