All penicillins inhibit the biosynthesis of the bacterial cell wall.
The penicillinase- resistant penicillins are active in vitro against a variety of other bacteria. One such procedure has been recommended for use with discs to test susceptibility to this class of drugs. Interpretations correlate diameters on the disc test with MIC values. A penicillinase- resistant class disc may be used to determine microbial susceptibility to cloxacillin, dicloxacillin, methicillin, nafcillin, and oxacillin. In general, all staphylococci should be tested against the penicillin G disc and against the methicillin disc.