Effects of Medium Composition on Penicillin-induced Hydrolysis and Loss of RNA and Culture Turbidity in Group A Streptococci

Exposure to penicillin G of exponentially growing cultures of group A streptococci growing in chemically defined medium (CDM) can lead to extensive loss of culture turbidity. Significant reductions in culture turbidity did not accompany comparable treatments of group A streptococci growing in Todd-Hewitt broth (THB). Studies with THB and a high-molecular-weight (greater than 12,000) fraction of THB demonstrated that components in this complex medium inhibited the efflux of RNA hydrolysis products from otherwise intact cells. Hydrolysis products accumulated intracellularly and inhibited the extensive hydrolysis of RNA and consequently the loss of culture turbidity. Results of survival studies with cultures of group A streptococci exposed to penicillin G in THB demonstrated that this treatment protocol produces conditions of phenotypic tolerance relative to exposure in CDM. In combination, these findings provide further support for the hypothesis of RNA hydrolysis as the bactericidal mechanism of penicillin G action in this nonlytic death phenotype.