A Test for Penicillin Sensitivity and Resistance in Staphylococcus.

Abstract

Measurement of penicillin sensitivity in staphylococci is influenced by the size of the inoculum on 2 accounts: (a) the presence in sensitive strains of a minority of resistant individuals originating by mutation; and (b) the occurrence of penicillinase-producing strains whose cells are individually sensitive to penicillin. A test for sensitivity and resistance is proposed, on the basis of comparative measurements of growth in solid and liquid media. The test records the presence or absence of growth in a series of liquid cultures with various penicillin concs. and various inocula: Sensitive strains give no visible growth in presence of 0.05 units/ml. of penicillin with medium or small inocula. Strains of higher resistance give results depending on the proportion of their cells that can grow in presence of various amts. of penicillin; these proportions are easily estimated from the results of the test. Penicillinase-producing strains often give results showing great discrepancies between large and small inocula, indicating a high degree of sensitivity of the individual cells.