Postantibiotic effect of penicillin on Streptococcus anginosus.

We studied the postantibiotic effect of penicillin G on bacterial growth of two strains of Streptococcus anginosus by optical density readings of the cultures and by counting the numbers of viable cells. Duration of the effect of the drug in concentrations equivalent to the MICs after exposure for 2 h was 3.4 and 3.5 h. The production of streptococcal substances was examined during the postantibiotic phase. The antibiotic caused an increase in deoxyribonuclease and a decrease in both free and cell-bound hemolysin activities of one strain. The other strain displayed an increase in hyaluronidase and both free and bound hemolysin production.