How partially inhibitory concentrations of chloramphenicol affect the growth of Escherichia coli

In the presence of up to 6 microM chloramphenicol, balanced exponential growth of Escherichia coli occurred at a reduced rate after an adjustment period. The inhibition of ribosome function by chloramphenicol within growing cells was inferred from measurements of growth rate and cell composition during balanced growth and of pulse-labeling of cells by radioactive proline after a 10-min exposure to chloramphenicol. In each case the results were consistent with simple noncompetitive inhibition of protein synthesis, with 50% inhibition occurring at 2 microM chloramphenicol, the concentration that gave 50% binding of chloramphenicol to purified ribosomes in vitro. The differences between these results and those obtained with cell-free protein synthesizing systems were shown to be in part due to slow binding of chloramphenicol and in part due to the slow rate and lack of saturation of the cell-free protein-synthesizing systems now available. During balanced growth in concentrations of chloramphenicol 1 microM or higher, the net rate of maturation of ribosomal ribonucleic acid was also inhibited (50% at 2.8 microM). The specific growth rate during balanced growth was inhibited by 50% at 1.8 microM chloramphenicol, but at higher concentrations inhibition was greater than expected from the simple noncompetitive dose-response observed for inhibition of polypeptide synthesis. However, the inhibition of maturation of ribosomal ribonucleic acid plus the inhibition of protein synthesis quantitatively accounted for the observed inhibition of growth. Thus, we have presented for the first time an essentially complete account of the effects of partially inhibitory concentrations of an antibiotic on the growth physiology of a bacterium.