Effect of penicillin on surface carbohydrate, hemolysin and morphology of Streptococcus pyogenes during and after the post-antibiotic phase.

The post-antibiotic effect (PAE) of penicillin was measured in vitro against a group A streptococcal strain by the kinetic growth method. The duration of the effect was 2.8 h. The bacterial morphology and some streptococcal products were analyzed during and after the PAE, after being exposed to penicillin in a concentration of 1xMIC for 2 h. Bacteria not previously exposed to penicillin were used as a control culture. Morphological changes and increases in the size of treated streptococci were observed by electronic microscope during the post-antibiotic phase. The post-penicillin effect on the production of cell-bound hemolysin and free hemolysin was examined using sheep red blood cells. Production of cell-bound hemolysin rose sharply, but was inhibited by the antimicrobial agent. The free lysin diminished significantly, and concomitantly with a higher production of free toxin by the treated cells. No effect was observed on the specific carbohydrate group when the antigen was tested with streptococcal group A antiserum.