Effects of benzylpenicillin on Streptococcus pyogenes during the postantibiotic phase in vitro.

A postantibiotic effect (PAE) for Streptococcus pyogenes M12, P1800 was induced by 10 x MIC of benzylpenicillin for 2 h in vitro. The PAE was found to be 2.4 h (range 1.75-3.0 h). No incorporation of 3H-thymidine by the streptococci occurred during the first hours of the postantibiotic phase (PA-phase), indicating that bacterial cell division was inhibited during this period. However, these bacteria showed the same killing rate as previously unexposed control cultures when 10 x MIC of benzylpenicillin was added during the PA-phase. When bacteria in PA-phase were re-exposed to penicillin at a concentration of 0.2 x MIC multiplication of the bacteria was inhibited for 6-7 h, while at a concentration of 0.3 x MIC slow killing was induced. In contrast, previously untreated controls exposed to 0.2 x MIC and 0.3 x MIC showed a growth rate corresponding to that of bacteria grown in the absence of penicillin. The effects of subinhibitory concentrations on streptococci previously exposed to penicillin may therefore be a more important factor in determining dosage intervals than the PAE.