Antimicrobial Efficacy of Gatifloxacin and Moxifloxacin with and without Benzalkonium Chloride Compared with Ciprofloxacin and Levofloxacin Against Methicillin- ResistantStaphylococcus aureus

We compared the antimicrobial activity of gatifloxacin and moxifloxacin with and without benzalkonium chloride (BAK) against clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA). Minimum inhibitory concentrations (MICs) against clinical isolates of MRSA were evaluated. Approximately 105 CFU/ml of methicillinresistant S. aureus was added to Mueller-Hinton broth containing two-fold concentration increments of drug. For the evaluation of gatifloxacin with BAK, 50 μg/ml of BAK were added to the first well of the plate with gatifloxacin or moxifloxacin and then serially diluted. The combination of gatifloxacin or moxifloxacin with BAK was more active than either fluoroquinolone without BAK. The MICs ranged from ≤0.008 μg/ml to 0.125 μg/ml for gatifloxacin plus BAK, from 0.063 μg/ml to ≥8 μg/ml with unpreserved gatifloxacin from <0.004 to 0.25 for moxifloxacin plus BAK, and from ≤0.016 μg/ml to 16.0 μg/ml with unpreserved moxifloxacin. The combinations of gatifloxacin or moxifloxacin and BAK were highly active against MRSA in vitro, providing MICs that were approximately 2- to 500-fold lower than the MICs provided by either gatifloxacin or moxifloxacin without BAK.