Effects of supra- and sub-MIC benzylpenicillin concentrations on group A beta-haemolytic streptococci during the postantibiotic phase in vivo.

A postantibiotic effect (PAE) in vivo was induced in group A streptococci established in a tissue cage model in rabbits. The bacteria were exposed to 10 x MIC of benzylpenicillin in tissue cage fluid (TCF) for 2 h. TCF was then aspirated, penicillin was eliminated by washing and the bacteria were transferred to tissue cages in other rabbits in order to study the in-vivo killing kinetics of streptococci in the postantibiotic phase. In these latter rabbits the concentration of benzylpenicillin in TCF corresponded to 10 or 0.3 x MIC. Bacteria not previously exposed to penicillin were used as controls. Streptococci in postantibiotic phase were killed as effectively after re-exposure to 10 x MIC in vivo as the growing controls. Although the concentration of penicillin fell below the MIC after 12 h, no regrowth was seen during the following 12 h in either culture. When only subinhibitory concentrations in TCF were used in the second phase, a killing of approximately 1 log10 cfu/ml was noted both in the previously exposed cultures and in controls. Both cultures started to multiply first after 6-7 h.