Identification of cephalosporin-resistant Staphylococcus aureus with the disc diffusion method

Abstract

Methicillin-resistant strains of Staphylococcus aureus, in total 84, representing 16 laboratories in 8 different countries were all resistant to 32 mug of cephalothin per ml with the same typical heteroresistant pattern. With the disc diffusion method, they were easily detected when cephalexin discs were used. With cephalothin discs, on the other hand, 26 to 49% would have been falsely categorized as Group I or II after 24 hr. It is recommended that susceptibility testing of S. aureus to cephalosporins by using the paper disc method be performed with 30-mug cephalexin discs on Mueller-Hinton agar without blood. With an inoculum of 10(6) bacteria/ml, an incubation temperature of 30 C, and an incubation time of 24 hr, a zone of less than 10 mm indicates presumptive heteroresistance. This corresponds to the international recommendation with a minimal inhibitory concentration of 32 mug/ml as the upper limit of Group II.