Reevaluation of the ability of the standardized disk diffusion test to detect methicillin-resistant strains of Staphylococcus aureus

Abstract

To reevaluate the ability of the disk diffusion method to detect methicillin-resistant Staphylococcus aureus, 73 such isolates from 13 cities were tested for antimicrobial susceptibility with the standardized disk diffusion test. Duplicate plates were incubated at 30 and 35°C and read after 18, 24, and 48 h. After incubation at 35°C for 24 h, 97% of isolates appeared resistant to methicillin, and 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 disks, whereas those from other cities had measurable zones of inhibition, with light growth inside the zones. Patterns of growth around cephalothin and cefamandole disks also varied among isolates from different cities. Incubation at 30°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° C. The results suggest that methicillin-resistant S. aureus strains from many areas will be detected if standardized disk diffusion tests are incubated at 35°C for 24 h.