Penicillinase resistant penicillins 

Penicillinase resistant penicillins are antibiotics, which are not inactivated by the penicillinase enzyme. Some bacteria produce the enzyme penicillinase that destroys the beta-lactam ring of the antibiotic, making the penicillin ineffective. Penicillinase resistant penicillins are used to treat resistant strains of staphylococci and other infections.

Penicillinase-resistant penicillins include meticillin, nafcillin and oxacillin. These are primarily used in the treatment of infection caused by penicillinase-producing staphylococci. Ampicillin was the first broad-spectrum penicillin and has a broader antibacterial range of action than that of penicillin G.

Penicillinase-sensitive penicillins, as their name suggests, are penicillins sensitive to cleavage by beta-lactamases, and important drugs of this group include Amoxicillin and Ampicillin.

Penicillinase destroys penicillin antibiotics. However, some penicillins, like flucloxacillin, are resistant to its effects making flucloxacillin suitable antibiotic therapy in infections caused by penicillinase producing staphylococci strains.