... penicillinase (now called ?-lactamase), an enzyme that easily breaks down penicillin. In contrast to 1941, when virtually every strain of S. aureus was susceptible to penicillin, today over 95% of S. aureus strains are resistant to penicillin, ampicillin, and the antipseudomonas penicillins (Neu 1065). Staphylococcus aureus can cause deadly infections of the skin, heart valves, blood, and bones, and in 1997, three Americans were reported to be infected with strains of it that were resistant to our last line of defense- the antibiotic vancomycin. Methicillin, a semisynthetic penicillin, was created to respond to this challenge. However, in the 1980s, methicillin-resitant S. aureus (MRSA) became a problem as well. Now, MRSA is resistant to all penicillins, cephalosporins, carbapenems, and penems due to a gene that produces a protein capable of bonding to and deactivating the penicillin (Neu 1065). Antibiotics are mostly derived from natural products. For example, penicillin is found in ...

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