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

  • Jorrit Mulder
  • Published 1996 in European Journal of Clinical Microbiology and Infectious Diseases

Abstract

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 μg methicillin disk on Mueller-Hinton agar using an incubation period of 48 h was compared with that of 1 (1 μg and 5 μg oxacillin disks on Mueller-Hinton agar with or without 2% NaCl, using an incubation period of 24 h. The detection ofmecA (MecAgen) by the polymerase chain reaction was used as a standard. Minimum inhibitory concentrations were determined by means of the E test. Of the 225mecA-positive strains, 190, 215, and 193 were resistant to 5 μg methicillin, 1 μg oxacillin and 5 μg oxacillin disks on Mueller-Hinton agar, respectively, and 216, 218, and 223 were resistant on Mueller-Hinton agar with 2% NaCl. Of the 94mecA-negative strains, 89, 93, and 94 were susceptible to 5 μg methicillin, 1 μg oxacillin, and 5 μg 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 μg/ml for oxacillin resistance and 8 μg/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 μg oxacillin disk was faster and easier to read than the methicillin disk and correlated better with detection ofmecA than the methicillin disk or the 1 μg oxacillin disk.

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