The Action of Penicillin on Staphylococcus: Further Observations on the Effect of a Short Exposure.

When a culture of Staphylococcus in the logarithmic period of growth is exposed to penicillin in appropriate concentration, growth of the bacteria is stopped promptly. If within a short time the penicillin is removed, no decrease in population of viable cells occurs, but, after a lag period, growth is resumed (Parker and Marsh, 1946). In earlier work it was shown that this effect occurred even though there was no detectable killing of organisms; it was not a peculiarity of the metabolic state of "persisters" (Bigger, 1944) in the culture. In the experiments to be detailed in this report, the observations have been extended to cover a total of 29 strains of Staphylococcus in order to determine whether the phenomenon was unique in the strain first observed or whether it was generally distributed. The strains were isolated from clinical material in a hospital laboratory and exhibited a wide range of penicillin sensitivity. Source and selection of cultures. In the course of a study of the relation between size of the test inoculum and apparent resistance to penicillin, all strains of Staphylococcus aureus isolated from clinical material in the routine laboratory serving Lakeside Hospital were subjected to tests of penicillin sensitivity (Parker, 1946). From the 169 cultures so studied, 29 were chosen, covering the range of sensitivity exhibited by the series. Test of sensitivity. The sensitivity test used was that described previously (Parker and Marsh, 1946; Parker, 1946). Serial dilutions of penicillin were prepared in nutrient broth; each was inoculated with 0.5 ml of a 106 dilution of an overnight broth culture. The tubes were incubated overnight, and the tube was then identified which contained just enough penicillin to prevent visible growth of bacteria. The final volume of the test was 1 ml. This was designated as the "penicillin sensitivity (small inoculum)" of the strain. Another test was made simultaneously, which differed only in that the inoculum contained 10,000 times as many organism, and the inhibitory penicillin concentration identified in this test was designated as "penicillin sensitivity (large inoculum)." Medium. The medium used was the tryptose broth supplied by the Digestive Ferments Laboratories; the pH after sterilization adjusted to 7.4 to 7.6.