Antibacterial activity, penicillinase stability and inducing ability of different penicillins.

There are now a t least three main types of antibacterial activity shown by different penicillins-the ordinary penicillin G type, the type shown by methicillin active against penicillinase-producing staphylococci and the. ‘ broad spectrum ’ type of activity shown by a-aminobenzylpenicillin. The inducing capacity of different penicillins can be measured by the concentration required for maximal or half maximal induction, which gives the Michaelis (induction) constant, or by the total amount of enzyme eventually produced. But either of these methods of determining inducing capacity may give misleading results with penicillins which may be easily hydrolysed during the induction process. Of the penicillins used methicillin is by far the best inducer-but it is uncertain how much this is due to a true ‘inducing affinity’ and how much a result of its resistance to hydrolysis.