Increased staphylococcal penicillinase activity accompanying penicillin treatment of experimentally infected mice.

In a previous communication, experiments were reported which indicate that the basal (constitutive) level of penicillinase in pathogenic penicillin resistant strains of Staphylococcus aureus can be inducibly elevated 10to 15-fold by in vitro exposure to penicillin under appropriate conditions (Geronimus and Cohen, 1957). If similar elevations in penicillinase activity are induced by penicillin treatment in vivo they must modify significantly the antibacterial effectiveness of penicillin. However, bacteria grown in an animal host may differ widely in enzymic and metabolic activities from bacteria of the same strain cultivated in vitro (Smith and Keppie, 1955; Segal and Bloch, 1956). Therefore, it was considered important to examine the properties of penicillinase in staphylocci taken directly from an animal host. This paper reports studies of penicillinase activity of staphylacocci isolated from the kidneys of experimentally infected mice.