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Penicillin-binding protein expression at different growth stages determines penicillin efficacy in vitro and in vivo:
D L Stevens1, S Yan, A E Bryant
1Department of Medicine, University of Washington School of Medicine, Seattle.
Abstract:
Mechanisms to explain the "inoculum effect" have not been elucidated in gram-positive infections. A mouse model of group A streptococcal myositis was used to compare the efficacies of two beta-lactams, penicillin 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 penicillin-binding protein (PBP) patterns from membrane proteins isolated from mid-log-phase and stationary-phase cultures of Streptococcus pyogenes were compared. Binding of radiolabeled penicillin by all PBPs was decreased in stationary cells; however, PBPs 1 and 4 were undetectable at 36 h. Thus, the loss of certain PBPs during stationary-phase growth in vitro may be responsible for the inoculum effect observed in vivo and may account for the failure of penicillin in both experimental and human cases of severe streptococcal infection.
Insights
The inoculum effect in Gram-positive infections is not well understood. This study found beta-lactams are more susceptible than clindamycin, potentially due to penicillin-binding protein changes in stationary-phase Streptococcus pyogenes.
Area of Science:
- Microbiology
- Infectious Diseases
- Pharmacology
Background:
- The "inoculum effect" describes reduced antibiotic efficacy with higher bacterial concentrations.
- Mechanisms underlying this phenomenon in Gram-positive infections, particularly Streptococcus pyogenes, remain unclear.
Purpose of the Study:
- To investigate the inoculum effect in a mouse model of Gram-positive bacterial myositis.
- To compare the in vitro and in vivo efficacy of beta-lactam antibiotics (penicillin, ceftriaxone) versus a protein synthesis inhibitor (clindamycin) across varying inoculum sizes.
Main Methods:
- Utilized a mouse model of group A streptococcal myositis.
- Assessed antibiotic efficacy (penicillin, ceftriaxone, clindamycin) at three distinct bacterial inoculum sizes.
- Compared penicillin-binding protein (PBP) patterns in mid-log-phase versus stationary-phase Streptococcus pyogenes cultures.
Main Results:
- Beta-lactam antibiotics demonstrated greater susceptibility to the inoculum effect compared to clindamycin, both in vitro and in vivo (P < .05).
- Stationary-phase Streptococcus pyogenes exhibited decreased binding of radiolabeled penicillin to PBPs, with PBPs 1 and 4 becoming undetectable at 36 hours.
Conclusions:
- Reduced penicillin-binding protein expression during stationary-phase growth may explain the observed inoculum effect in Gram-positive infections.
- This PBP alteration could contribute to the clinical failure of penicillin in severe streptococcal infections.