PENICILLIN-BINDING PROTEIN EXPRESSION AT DIFFERENT GROWTH-STAGES DETERMINES PENICILLIN EFFICACY INVITRO AND INVIVO - AN EXPLANATION FOR THEINOCULUM EFFECT

Mechanisms to explain the ''inoculum effect'' have not been elucidated in gram-positive infections. A mouse model of group A streptococcal m yositis was used to compare the efficacies of two beta-lactams, penici llin and ceftriaxone, and a protein synthesis inhibitor, clindamycin, at three different inoculum sizes. Beta-lactams were more susceptible to inoculum effects than was clindamycin both in vivo and in vitro (P < .05). The large inocula were hypothesized to reach stationary phase of growth sooner than smaller inocula both in vitro and in vivo. The p enicillin-binding protein (PBP) patterns from membrane proteins isolat ed from mid-log-phase and stationary-phase cultures of Streplococcus p yogenes were compared. Binding of radiolabeled penicillin by all PBPs was decreased in stationary cells; however, PBPs 1 and 4 were undetect able at 36 h. Thus, the loss of certain PBPs during stationary-phase g rowth in vitro may be responsible for the inoculum effect observed in vivo and may account for the failure of penicillin in both experimenta l and human cases of severe streptococcal infection.