Penicillinase Production in the Evaluation of Disk Sensitivity Testing of Staphylococci to Penicillin

Abstract

The production of penicillinase by 262 strains of staphylococci was used as a basis for evaluating the results of testing for penicillin sensitivity by the disk diffusion technic. Presence of penicillinase was determined after induction by methicillin using N-phenyl-naphthylamineazo- o-carboxybenzyne (PNCB). All 133 strains of S. aureus and most of the 129 strains of S. epidermidis could be readily separated into resistant and sensitive groups (penicillinase producers and nonpenicillinase producers) on the basis of sizes of zones of inhibition in the disk test. It was observed that many strains of both species of staphylococci, although capable of producing penicillinase, had growth patterns with wide zones of inhibition around disks containing two units of penicillin. With some strains of S. epidermis induction of penicillinase production was not always easily effected, and resistance appeared to depend upon factors other than penicillinase production.