Beta-lactamase testing of staphylococci in a commercial broth microdilution minimum inhibitory concentration system

Beta-Lactamase testing of clinical isolates of staphylococci may be performed in commercial broth microdilution minimum inhibitory concentration plates, using a chromogenic cephalosporin reagent directly in a well containing a noninhibitory concentration of a semisynthetic penicillin which serves as an inducer. A total of 115 staphylococcal isolates tested in 0.25 to 0.50 microgram of methicillin per ml in Mueller-Hinton broth showed 100% correlation with beta-lactamase tests performed on Mueller-Hinton agar, using 1-microgram oxacillin disk as an inducer.