penicillinase-resistant antibiotic, an antimicrobial agent that is not rendered inactive by penicillinase, an enzyme produced by certain bacteria, especially by strains of staphylococci. The semisynthetic penicillins cloxacillin sodium, dicloxacillin sodium, methicillin sodium, nafcillin sodium, and oxacillin sodium resist the action of penicillinase and are used in treating infections caused by staphylococci that elaborate the enzyme.
