Inactivation of Methicillin, Oxacillin and Ancillin by Staphylococcus aureus.

Each of the 3 semisynthetic penicillins–ancillin, methicillin and oxacillin–was studied for inactivation by a penicillinase-producing strain of Staphylococcus aureus. Starting with 108 viable staphylococcal units per ml and a concentration of each penicillin 15 times the minimum inhibiting (and bactericidal) concentration (MIC) for 105 organisms per ml in the same medium, these penicillins were inactivated at 37°C at a much more rapid rate than in the same medium at the same temperature without organism, or with the same number of organisms at 4°C, or in a non-nutrient medium (buffer, pH 6.4) at 37°C. The inactivation of each of these penicillins was accompanied first by a steady fall in the number of viable staphylococci until the concentration of drug fell below the “usual MIC,' after which there was demonstrable multiplication of the organisms. The organisms that survived and multiplied were no less susceptible than the parent strain to the action of the penicillin to which they had been exposed.