Use of a DNA hybridization method to verify results of screening for methicillin resistance in Staphylococci

Tests were performed by the disk diffusion method, agar dilution method and the E test to determine the susceptibility to methicillin and oxacillin of clinical isolates and control strains ofStaphylococcus aureus (n=106) and coagulase-negative species (n=131). Results were compared with those of a dot blot DNA hybridization test, in which themecA gene was detected using an oligonucleotide probe selected from themecA gene. Among theStaphylococcus aureus strains themecA gene was found in all but two strains inhibited by 8 mg/l of methicillin and all but two strains inhibited by 4 mg/l of oxacillin. A disk test using either 1 µg oxacillin or 10 µg methicillin and a tentative resistance breakpoint of 10 mm gave the best agreement with the hybridization test. For coagulase-negative staphylococci 34 of 35 strains inhibited by 8 mg/l methicillin hybridized with the probe as well as 58 of 82 strains inhibited by 1–4 mg/l; 93 of 97 strains inhibited by 0.5 mg/l oxacillin were also positive in the probe test. Using the 1 µg oxacillin disk and a resistance breakpoint of 10 mm good agreement was obtained between results of the disk diffusion and DNA hybridization tests. It is suggested that this genotypic method for detection of methicillin resistance is used as a reference method for routine methods.