Evaluation of different methods for the detection of methicillin resistance in coagulase-negative staphylococci

The efficacy of 19 agar diffusion methods for the detection of methicillin resistance among coagulase-negative staphylococci (CoNS) within 24 h was evaluated. A total of 359 CoNS isolates were tested, of which 204 were Staphylococcus epidermidis. In 164 isolates, the presence of mecA was investigated; 61 strains were mecA-positive and 103 were mecA-negative by Southern blot analysis. Based on the best agreement shown with the mecA determination (94%) among four agar dilution assays for determining methicillin MIC, an assay with Columbia agar supplemented with NaCl and incubation with a heavy bacterial inoculum of 10(5)-10(6) cfu/spot was used as the reference MIC method. The best agar diffusion results were obtained with a 1 microg oxacillin disc on Columbia agar with 4.5% NaCl supplement. With this method, 99% of S. epidermidis and 94% of non-S. epidermidis were in agreement with the MIC determination. However, Columbia (without NaCl), Mueller-Hinton and Isosensitest agars were almost as useful when a 1 microg oxacillin disc was used. The zone breakpoints for S. epidermidis were, in general, considerably larger than those for other CoNS species and, consequently, differentiation according to species is recommended. Furthermore, resistance to other antibiotics, such as gentamicin and erythromycin, makes methicillin resistance highly likely.