Kill rates of preserved and preservative-free topical 8-methoxy fluoroquinolones against various strains of Staphylococcus

Purpose: To assess the kill rates of preserved and preservative-free 8-methoxy fluoroquinolones and benzalkonium chloride (BAC) against isolates of Staphylococcus aureus and coagulase-negative Staphylococcus (CNS) species. Setting: Bascom Palmer Eye Institute, Miami, Florida, USA. Methods: Ocular and standard isolates of S aureus and CNS were inoculated with saline, 0.005% BAC, gatifloxacin 0.3% containing 0.005% BAC (Zymar), or BAC-free moxifloxacin 0.5% (Vigamox) at 37°C. Bacterial viability was assessed after 15 minutes, 30 minutes, and 60 minutes. Results: Thirteen isolates of S aureus and 5 isolates of CNS were used. The mean initial load of bacterial inoculum was 5.45 log colony-forming units (CFU)/mL ± 0.37 (SD). Saline did not affect the density of staphylococci; BAC significantly decreased the staphylococci count to a mean of 3.80 ± 0.32 log CFU/mL at 60 minutes (P<.05). Gatifloxacin 0.3% reduced the number of staphylococci significantly more than moxifloxacin 0.5% at 15 minutes (mean 0.47 ± 1.12 log CFU/mL versus 4.55 ± 0.60 log CFU/mL), 30 minutes (mean 0.07 ± 0.31 log CFU/mL versus 3.82 ± 1.07 log CFU/mL), and 60 minutes (mean 0.00 ± 0.00 log CFU/mL versus 2.75 ± 1.29 log CFU/mL) (P<.005). Gatifloxacin 0.3% completely eradicated most S aureus (10/13) and CNS (3/5) isolates at 15 minutes; moxifloxacin 0.5% did not achieve complete kill in any S aureus isolate (13/13) or in most CNS isolates (4/5) at 60 minutes. Conclusion: The commercial formulation of gatifloxacin 0.3% containing BAC 0.005% completely eradicated staphylococcal isolates more rapidly than the BAC-free formulation of moxifloxacin 0.5% or BAC 0.005% alone.