Effect of growth phase on the bactericidal action of chloramphenicol against Haemophilus influenzae type b and Escherichia coli K-1.

The bactericidal action of chloramphenicol against six Haemophilus influenzae type b isolates and six Escherichia coli K-1 isolates was compared. Cells were grown in antibiotic-free medium into the late-stationary and mid-exponential phases of growth, and inocula of 10(5) to 10(6) cells per ml were added to fresh media containing 1 or 10 micrograms of chloramphenicol per ml for H. influenzae isolates, 80 micrograms of chloramphenicol per ml for E. coli isolates, or no chloramphenicol (antibiotic free). Quantitative kinetic studies indicated that each chloramphenicol concentration killed H. influenzae cells in the stationary phase of growth significantly more rapidly than it did those in the exponential phase of growth (P less than 0.001; analysis of variance). E. coli in either the stationary or the exponential phase were killed at the same rate by 80 micrograms of chloramphenicol per ml (P greater than 0.05). These results suggest that chloramphenicol may kill these organisms by different mechanisms.