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 metho d and the E test to determine the susceptibility to methicillin and ox acillin of clinical isolates and control strains of Staphylococcus aur eus (n = 106) and coagulase-negative species (n = 131). Results were c ompared with those of a dot blot DNA hybridization test, in which the mecA gene was detected using an oligonucleotide probe selected from th e mecA gene. Among the Staphylococcus aureus strains the mecA gene was found in all but two strains inhibited by greater-than-or-equal-to 8 mg/l of methicillin and all but two strains inhibited by greater-than- or-equal-to 4 mg/l of oxacillin. A disk test using either 1 mug oxacil lin or 10 mug methicillin and a tentative resistance breakpoint of les s-than-or-equal-to 10 mm gave the best agreement with the hybridizatio n test. For coagulase-negative staphylococci 34 of 35 strains inhibite d by greater-than-or-equal-to 8 mg/l methicillin hybridized with the p robe as well as 58 of 82 strains inhibited by 1-4 mg/l; 93 of 97 strai ns inhibited by greater-than-or-equal-to 0.5 mg/l oxacillin were also positive in the probe test. Using the 1 pg oxacillin disk and a resist ance breakpoint of less-than-or-equal-to 10 mm good agreement was obta ined between results of the disk diffusion and DNA hybridization tests . It is suggested that this genotypic method for detection of methicil lin resistance is used as a reference method for routine methods.