Oxacillin susceptibility of coagulase-negative staphylococci: role for mecA genotyping and E-test susceptibility testing.

For 115 clinical isolates of coagulase-negative staphylococci which were acquired from 100 patients, we compared the results of routine critical agar dilution susceptibility testing (4% NaCl, 6 mg/ml oxacillin) with mecA gene detection by a polymerase chain reaction amplification. Discrepant results were subsequently reassessed with critical agar dilution testing, repeat mecA genotyping, disc radial diffusion susceptibility testing, E-test and determination of beta-lactamase status. For the initial comparisons, 36 isolates were susceptible and lacked mecA whereas 54 isolates were resistant and had evidence of mecA. Among 17 mecA positive/agar dilution susceptible bacteria, four were clearly resistant by E-test, one isolate had borderline resistance (MIC, 2-4 mg/l), and 12 were resistant when E-test was applied to resistant subpopulations. Initial E-tests facilitated the recognition of the heteroresistant isolates. For eight mecA negative/agar dilution resistant isolates, two were confirmed as oxacillin susceptible and six were mecA positive upon retesting. Although 53.9% had been classified initially as resistant by agar dilution, 67.0% were finally deemed resistant. Critical agar dilution underestimates oxacillin resistance among coagulase-negative staphylococci, and accurate detection of resistance is facilitated by mecA genotyping and E-test.