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S. D. Borsos, J. M. Blondeau, C. K. Hesje; Comparative in vitro Potency of Levofloxacin (Lfx) and Gatifloxacin (Gfx) Plus Benzalkonium Chloride (BAK) Against Gram-Positive (GP) Pathogens Including Methicillin-Resistant Staphylococcus aureus (MRSA). Invest. Ophthalmol. Vis. Sci. 2008;49(13):1980.
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Fluoroquinolones are commonly used for treating ocular infections; however, they are not equivalent in terms of their in vitro activity against GP pathogens. Additionally, BAK has been previously shown to contribute to the in vitro potency of Gfx against GP pathogens. We compared the in vitro activity of Lfx to that of Gfx and Gfx plus BAK against GP pathogens.
Minimum inhibitory concentration (MIC) testing was based on current Clinical and Laboratory Standards recommendations. Coagulase-negative staphylococci (CNS) (n = 20), methicillin-susceptible S aureus (MSSA) (n = 20), and MRSA (n = 20) were incubated at the density of 105 CFU/mL in media containing 2-fold concentration increments of the study drugs. When present, BAK was added at concentrations from 3.125 µg/mL to 6.25 µg/mL. Following incubation under optimum conditions, the lowest drug concentration preventing growth of 50% and 90% of bacteria was recorded as the MIC50 and MIC90, respectively.
The MICs of study drugs against GP pathogens are presented in the Table. Gfx had 2-4-fold lower MICs than Lfx against CNS, MSSA, and MRSA. The MICs of Gfx plus BAK were 8-2000-fold lower than those of Lfx and 4-1000-fold lower than those of Gfx alone.
Gfx plus BAK was substantially more potent than Lfx and Gfx alone against CNS, MSSA, and MRSA. These findings suggest that Zymar®, an ophthalmic solution of gatifloxacin containing BAK (50 µg/mL), is highly active against GP pathogens including multidrug-resistant strains.
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