In vitro pharmacodynamics of ofloxacin and ciprofloxacin against common ocular pathogens

Purpose. Time-kill curve methodology was used to assess the pharmacodynamics of two fluoroquinolones, ofloxacin and ciprofloxacin, against six strains representing the most common ocular pathogens: Pseudomonas aeruginosa, Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus pneumoniae, Serratia marcescens, and Haemophilus influenzae. Methods. For time-kill studies, ofloxacin and ciprofloxacin solutions were prepared at concentrations of 0.5 x, 1.0 x, 2.0 x, and 3.0 x the MIC (minimal inhibitory concentration) for each respective strain. Inocula were prepared by diluting overnight cultures to final concentrations of 10(2), 10(3), 10(4), and 10(5) cfu (colony-forming units)/mL in each antibiotic solution. Growth controls were included. Viability counts of antibiotic-containing and control bacterial suspensions were performed at 0, 10, 20, 30, 60, 90, 120, and 180 minutes. Results. In general, the kill rates of ofloxacin at 1.0 x, 2.0 x, and 3.0 x the MIC were significantly faster than the kill rates of ciprofloxacin by approximately 30 minutes, regardless of the bacterial concentration tested. At 0.5 x MIC, the kill kinetics of ciprofloxacin and ofloxacin were similar, regardless of the strain tested. At 1.0 x MIC, ofloxacin achieved 99.9% killing of P. aeruginosa and S. aureus within 30 minutes, and S. epidermidis and S. marcescens within 90 minutes. Overall, the kill kinetics of both quinolones for H. influenzae were similar, while neither quinolone achieved 99.9% killing of S. pneumoniae, regardless of the antibiotic concentration tested. Conclusion. Time-kill curve analyses in the present study demonstrate that ofloxacin achieved killing of the majority of ocular pathogens tested at rates equivalent to or faster than that of ciprofloxacin. Both fluoroquinolones were more effective against nonencapsulated bacteria than against encapsulated bacteria.