Evaluation of oxacillin tolerance in Staphylococcus aureus by a novel method

A novel agar dilution plate-count procedure for the quantitative measurement of bacterial inhibition and killing is described. For Staphylococcus aureus versus oxacillin, by the agar dilution plate-count procedure it was found that only 1 of 20 clinical isolates and 1 of 7 allegedly tolerant reference isolates met the conventional definition of tolerance. By using inocula of 10(5) CFU per plate, most isolates were demonstrated to have subpopulations of cells which, although inhibited, persisted for 24 h in concentrations significantly above their MICs. The persister percentages at 24 h appeared to be strain dependent, and all persisters exhibited the paradoxical effect. For each isolate, the persister percentage markedly decreased after action by oxacillin for 48 h, and the paradoxical effect was greatly diminished. Our findings suggest that tolerance is an artificial and arbitrary concept that does not adequately characterize the inhibition and killing dynamics associated with the persister phenomenon.