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.
6 Figures and Tables
Figure 1: Four phenotypes of sensitivity. A, phenotype S; B, phenotypes R1 and R2; C, phenotype R3.
Table 1" Distribution of MecA in 319 strains of coagulase-negative staphylococci.
Table 2: Presence of mecA in 319 strains tested on Mueller-Hinton agar with and without 2% NaCI, using 5 IJg methicillin, 1 pg oxacillin and 5 IJg oxacillin disks.
Figure 2: Histogram of inhibition zone diameters obtained with 5 txg methicillin, 1 ixg oxacillin, and 5 ixg oxacitlin disks on Mueiler-Hinton agar (A, B, C) and on Mueller-Hinton agar plus 2% NaCI (D, E, F). Black bars indicate mecA-positive strains.
Figure 3: Histogram of MICs determined by the E test for methicillin (A) and oxaciltin (B) after 48 h of incubation. Black bars indicate m e c A - p o s i t i v e strains.
Table 3" Distribution of phenotypes R2 and R3 in 225 m e c A - p o s i t i v e strains of coagulase-negative staphylococci, according to the different disk tests.
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