COMPARISON OF PHENOTYPIC METHODS AND DNA HYBRIDIZATION FOR DETECTION OF METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS

One hundred thirty-eight Staphylococcus aureus isolates from patients with severe staphylococcal infections were collected in 15 French hosp itals. Detection of the mec gene was performed by dot blot hybridizati on with a specific DNA probe. Dot blot results were used to characteri ze the isolates as methicillin susceptible (77 isolates) or resistant (61 isolates). The isolates were screened for methicillin resistance b y an agar spread method on Mueller-Hinton plates containing oxacillim (2 and 10 mu g/ml) and were incubated at 37 degrees C, with 10(8) CFU as the inoculum. MICs of oxacillin and methicillin were determined by the agar dilution method on Mueller-Hinton plates without NaCl, by usi ng 10(5) CFU per spot, after 24 and 48 h of incubation at 30 or 37 deg rees C. Moderately elevated MICs were found for 20 isolates (14.5%). T he mec gene was detected in six (30%) of the isolates expressing a low level of resistance to methicillin and/or oxacillin. As determined by comparison with probe hybridization results, the spread plate method with oxacillin at 2 mu g/ml was more sensitive (sensitivity, 100%) and specific (specificity, 100%) than agar dilution with either methicill in or oxacillin in identifying methicillin resistance or susceptibilit y Determinations of methicillin and oxacillin MICs by the agar dilutio n method had a specificity of 99 to 100% depending on the conditions o f incubation, but the sensitivity was below 85% whatever the duration or temperature of incubation.