Summary auto-generated
This study examined how bacterial growth rate affects the bactericidal activity of β-lactam antibiotics against Escherichia coli. Researchers used chemostat cultures to maintain bacteria at controlled, slow generation times (0.7–12 hours) and measured killing rates by two β-lactams: cefonicid and CGP 17520. The key finding was that while killing rates varied dramatically in absolute time across different growth rates, the killing rate remained constant when expressed per bacterial generation. Rapidly growing cells (40-minute generation time) were killed in approximately 1.2 generations, as were slowly growing cells with 12.6-hour generation times. Cell wall degradation rates similarly correlated with generation time. Penicillin-binding protein (PBP) analysis revealed decreased binding to PBPs 1a/b in slowly growing bacteria, potentially explaining the growth rate dependence. The authors conclude that slowly growing bacteria exhibit phenotypic tolerance to β-lactams proportional to their reduced growth rate, with important implications for antibiotic therapy in clinical infections where pathogens grow slowly.
Key findings
- The rate of bacterial killing by β-lactam antibiotics is directly proportional to generation time: slowly growing bacteria die more slowly in absolute time but at constant rates per generation
- Both bacterial killing and cell wall lysis show the same generation-time-dependent relationship across multiple β-lactam drugs and E. coli strains
- Penicillin-binding protein 1a/b levels decrease with slower growth rates, correlating with reduced antibiotic susceptibility
- Different β-lactam drugs and bacterial strains show different absolute killing rates per generation, but the proportionality to generation time remains constant
- Non-growing bacteria exhibit complete phenotypic tolerance to β-lactams, suggesting a spectrum of tolerance from stationary phase through various slow-growth conditions
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Abstract
Summary: Nongrowing bacteria evade the bactericidal activity of βbT-lactam antibiotics. We sought to determine if slow growth rate also alters bactericidal activity. The bactericidal activity of two βbT-lactams on Escherichia coli grown in glucose limited chemostats was compared for generation times ranging from 0·7 to 12 h. The degree of killing varied with drug structure and with E. coli strain. However, all killing rates were a constant function of the bacterial generation time: slowly growing bacteria became progressively more phenotypically tolerant to βbT-lactam antibiotics as the generation time was extended.