REEVALUATION OF THE ABILITY OF THE STANDARDIZED DISK DIFFUSION TEST TO DETECT METHICILLIN-RESISTANT STRAINS OF STAPHYLOCOCCUS-AUREUS

To reevaluate the ability of the disc diffusion method to detect methicillin-resistant S. aureus, 73 such isolates from 13 cities were tested for antimicrobial susceptibility with the standardized disc diffusion test. Duplicate plates were incubated at 30.degree. and 35.degree. C and read after 18, 24 and 48 h. After incubation at 35.degree. C for 24 h, 97% of isolates appeared resistant to methicillin; 99% appeared resistant to oxacillin. A significantly smaller proportion of isolates appeared resistant to cephalothin (P < 0.001) and cefamandole (P < 0.001). Isolates from some cities had no zones of inhibition around methicillin and oxacillin discs; those from other cities had measurable zones of inhibition, with light growth inside the zones. Patterns of growth around cephalothin and cefamandole discs also varied among isolates from different cities. Incubation at 30.degree. C for 24 h did not result in better detection of methicillin or oxacillin resistance. All study isolates appeared resistant to methicillin and oxacillin after 48 h of incubation at 35.degree. C. Evidently, methicillin-resistant S. aureus strains from many areas will be detected if standardized disc diffusion tests are incubated at 35.degree. C for 24 h.