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 methici llin resistance among coagulase-negative staphylococci (CoNS) within 2 4 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 MIG, 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 diffus ion results were obtained with a 1 mu g oxacillin disc on Columbia aga r with 4.5% NaCl supplement. With this method, 99% of S. epidermidis a nd 94% of non-S. epidermidis were in agreement with the MIC determinat ion. However, Columbia (without NaCl), Mueller-Hinton and Isosensitest agars were almost as useful when a 1 mu g oxacillin disc was used. Th e zone breakpoints for S. epidermidis were, in general, considerably l arger than those for other CoNS species and, consequently, differentia tion according to species is recommended. Furthermore, resistance to o ther antibiotics, such as gentamicin and erythromycin, makes methicill in resistance highly likely.