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SUMMARYThe induction of penicillinase in Staphylococcus aureus resistant to benzyl penicillin has been investigated. Inducers tested included several new penicillins derived from 6-aminopenicillanic acid, benzyl penicillin amide and cephalosporin C. Fifteen strains of S . aureus were examined, one in detail and the others using only two or three inducers. All strains possessed basal penicillinase and all were inducible, although maximum enzyme levels varied somewhat from one strain to another.Hydrolysis of the penicillins by staphylococcal penicillinase was investigated by using supernatant fluid from a cephalosporin C-induced culture of Staphylococcus aureus 524 as the source of enzyme. Under these conditions all the penicillins except methicillin were inactivated, although a t different rates. Hydrolysis of methicillin was only detectable a t much higher enzyme concentrations. Hydrolysis of cephalosporin C was not observed.The use of penicillins for the treatment of staphylococcal infections is discussed in the light of these results.

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