Degree and stability of tolerance to penicillin in Streptococcus pyogenes.

The degree of antibiotic tolerance may be assessed by determining the tolerance percentage of a bacterial strain, defined as the surviving fraction of an inoculum that has been exposed for 24 hours to a high concentration of a beta-lactam antibiotic. In 61 clinical isolates of Streptococcus pyogenes, tolerance percentages ranged from 0 to 0.43. From the slopes of the killing curves it can be deduced that killing starts to be delayed at a tolerance percentage of 0.1. Although a tolerance percentage exceeding 0.1 was observed in 41.4% of the strains, the incidence of clinically relevant forms of tolerance is expected to occur in a smaller fraction of the strains. Tolerance percentages of two strains stored at 20 degrees C, 4 degrees C or -70 degrees C (tolerance percentages 0.43 and 0.36) decreased to 0.03 or less in six weeks. Tolerance percentages could be completely restored in these strains, but not in sensitive strains, by successive selection for this property on penicillin gradients of increasing concentration. In four strains isolated from a family outbreak, identical levels of tolerance percentage could be selected for with the same technique.