Comparison of disk diffusion, the E test, and detection of mecA for determination of methicillin resistance in coagulase-negative staphylococci.

The aim of this study was to find a reliable, fast, and simple alternative to the methicillin disk method for determination of methicillin resistance in coagulase-negative staphylococci, since results of this method are often difficult to read due to growth within the zone of inhibition. The sensitivity of 319 strains of coagulase-negative staphylococci to a 5 microgram methicillin disk on Mueller-Hinton agar using an incubation period of 48 h was compared with that of 1 microgram and 5 micrograms oxacillin disks on Mueller-Hinton agar with or without 2% NaCl, using an incubation period of 24 h. The detection of mecA (MecAgen) by the polymerase chain reaction was used as a standard. Minimum inhibitory concentrations were determined by means of the E test. Of the 225 mecA-positive strains, 190, 215, and 193 were resistant to 5 micrograms methicillin, 1 microgram oxacillin and 5 micrograms oxacillin disks on Mueller-Hinton agar, respectively, and 216, 218, and 223 were resistant on Mueller-Hinton agar with 2% NaCl. Of the 94 mecA-negative strains, 89, 93, and 94 were susceptible to 5 micrograms methicillin, 1 microgram oxacillin, and 5 micrograms oxacillin disks on Mueller-Hinton agar, respectively, and 92, 93, and 94 were susceptible on Mueller-Hinton agar with 2% NaCl. Using breakpoints of 2 micrograms/ml for oxacillin resistance and 8 micrograms/ml for methicillin resistance, the E test yielded sensitivities of 99.6 and 99.1% and specificities of 97.9 and 98.9% after 48 h of incubation. The 5 microgram oxacillin disk was faster and easier to read than the methicillin disk and correlated better with detection of mecA than the methicillin disk of the 1 microgram oxacillin disk.