Streptococcus pneumoniae killing rate and post-antibiotic effect of levofloxacin and ciprofloxacin.

In vitro killing rates for levofloxacin and ciprofloxacin for six strains of Streptococcus pneumoniae were determined by the time-kill method outlined by the NCCLS. Both drugs were bactericidal at concentrations of two and four times their respective minimum inhibitory concentrations (MICs). Levofloxacin achieved a 99.9% kill on average in 1 hour more rapidly than ciprofloxacin. Post-antibiotic effect was also determined for both drugs against the same six strains. A post-antibiotic effect for a mean of 1.2 and 1.0 hours was observed for levofloxacin and ciprofloxacin, respectively. The latter means were not considered significantly different.