Evaluation of the in vitro bactericidal action of ciprofloxacin on cells of Escherichia coli in the logarithmic and stationary phases of growth.

Cells of Escherichia coli Neumann and E. coli KL16 were suspended in phosphate-buffered saline pH 7.4 and allowed to reach stationary growth conditions. Ciprofloxacin was added at different concentrations, and time-kill curves were constructed. It could be demonstrated that the number of viable cells was reduced quickly by several logs for E. coli Neumann, whereas a weak and slow killing effect was observed with E. coli KL16. When ciprofloxacin or norfloxacin was added to logarithmically growing cultures of E. coli Neumann or E. coli KL16, no principal differences in the killing rate for the two strains could be observed. Ciprofloxacin, however, was more bactericidal than norfloxacin. It was also demonstrated that the bactericidal action of ciprofloxacin on cells in the stationary growth phase was better at pH 7.4 than at pH 8.6. This dependence is different from that observed in MIC studies, in which the MIC were lower at pH 8.0 than at pH 7.2. It was also found that the bactericidal action of ciprofloxacin or norfloxacin on cells of E. coli Neumann in the stationary phase of growth could not be reduced by the addition of chloramphenicol, whereas under conditions of logarithmic growth the rapid killing effect of ciprofloxacin was reduced in the presence of chloramphenicol.