Detection of staphylococcal resistance to penicillinase-resistant penicillins. A comparison of two widely used disk...

One hundred Staphylococcus spp. strains were used to compare the results of two single potency disk diffusion methods (oxacillin and methicillin disks). The NCCLS disk diffusion and dilution methods correctly categorized all strains resistant to penicillinase-resistant penicillins (PRPs). The SHOWA disk method (Showa Yakuhin Kako Co., Ltd., Japan) produced false-susceptible results with approximately half of the PRP-resistant strains. The errors by the SHOWA method seemed to be the result of an excessive amount of drug (30 micrograms) in the disks, a light inoculum, and a high susceptible breakpoint (less than or equal to 25-50 micrograms/ml). Until these and other major technical differences between these two methods are resolved by internationally accepted testing standards, susceptibility statistics, and/or clinical trials, information from the United States and Japan cannot be reliably compared.