An in vitro resistance study of levofloxacin, ciprofloxacin, and ofloxacinusing keratitis isolates of Staphylococcus aureus and Pseudomonas aeruginosa

Purpose: We compared levofloxacin with ciprofloxacin and ofloxacin using th e in vitro susceptibilities of Staphylococcus aureus (SA) and Pseudomonas a eruginosa (PA) keratitis isolates. Design: Retrospective, clinical laboratory study of antibiotic susceptibili ty among keratitis isolates. Participants. Keratitis isolates from 200 patients with either SA or PA ker atitis. Methods: Minimum inhibitory concentrations (MICs) were determined for levof loxacin, ofloxacin, and ciprofloxacin for 93 SA keratitis isolates (68 fluo roquinolone-resistant and 25 susceptible, as determined by disk diffusion) and 107 PA keratitis isolates (13 fluoroquinolone-resistant and 94 suscepti ble). National Committee for Clinical Laboratory Standards susceptibilities were determined and analyzed statistically. Time kill studies were determi ned for fluoroquinolone-susceptible and -resistant isolates to all antibiot ics at 8 mug/ml. The killing rates were determined by regression, and the c olony count decreases were analyzed. Main Outcome Measures: The susceptibilities and potencies of levofloxacin, ciprofloxacin, and ofloxacin to SA and PA were determined from the MICs. Ti me kill studies determined the killing rates and decreases in colony counts . Results: The fluoroquinolone-resistant SA susceptibilities to levofloxacin, ofloxacin, and ciprofloxacin were only 22%, 10%, and 3%, respectively. The fluoroquinolone-susceptible SA were 100% susceptible to all antibiotics, w ith levofloxacin demonstrating the best potency. The fluoroquinolone-resist ant PA were resistant to all antibiotics. The fluoroquinolone-susceptible P A isolates were highly susceptible to levofloxacin, ofloxacin, and ciproflo xacin, with ciprofloxacin demonstrating the highest potency. For fluoroquin olone-susceptible SA and PA, the time kill studies determined that the kill ing rates and decreases in colony counts were equivalent for all three anti biotics tested. The time kill studies demonstrated no colony count decrease s for the fluoroquinolone-resistant SA and PA. Conclusions: Taken together, our susceptibility and time kill data failed t o demonstrate convincing differences in the susceptibility of SA and PA ker atitis isolates to levofloxacin, ciprofloxacin, and ofloxacin. In general, bacterial isolates that were resistant to ciprofloxacin and ofloxacin were also resistant to levofloxacin. Ophthalmology 2001;108:1826-1829 (C) 2001 b y the American Academy of Ophthalmology.