A method for the assay of penicillin

Abstract

A more detailed account is given of a previously described method which has been in routine use for 4 yrs. and can be applied to some other antibiotics. Open-ended glass or porcelain cylinders are placed on an agar plate seeded with a sensitive test organism. Unknown solns. (which need not be sterile) and standard solns. containing known amts. of penicillin are placed in the cylinders and the plates are incubated. The agar becomes covered by a confluent growth of bacteria except for a circular zone around each cylinder where the penicillin has diffused out. The diam. of the zone of inhibition varies with the conc. of the inhibitor, and the strength of the unknown soln. can be expressed in terms of the standard. The method is largely empirical; quantitative information on the limits of accuracy is given and advantages, applications and limitations of the method are discussed.