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C. K. Hesje, J. M. Blondeau; Impact of Benzalkonium Chloride (BAK) Dilution on Minimum Inhibitory Concentrations (MICs) of Gatifloxacin (Gfx) Against Methicillin-Resistant Staphylococcus aureus (MRSA) and Other Gram-Positive (GP) Pathogens. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1981.
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The presence of BAK in Zymar® 0.3% was previously shown to reduce the MIC of its active ingredient, gatifloxacin, against GP pathogens in vitro. We assessed the impact of BAK dilution in tear on the MIC of Gfx against GP pathogens.
Coagulase-negative staphylococci (CNS) (n = 20), MRSA (n = 20), and methicillin- susceptible S aureus (MSSA) (n = 20) were incubated at the density of 105 CFU/mL in media containing 2-fold concentration increments of Gfx. BAK was added in doubling dilutions from 50 µg/mL to 0.049 µg/mL. Following incubation, the lowest concentration preventing growth was determined as the MIC.
The MIC of gatifloxacin against all CNS, MRSA, and MSSA strains decreased by at least 8-500-fold in the presence of 3.125 µg/mL or 6.250 µg/mL of BAK (Table). The lowest BAK concentration that decreased the MIC of Gfx to ≤ 0.008 µg/mL against most strains of CNS (17/20), MRSA (19/20), and MSSA (18/20) was 3.125 µg/mL for CNS and MRSA and 1.57 µg/mL for MSSA. The additive effect of BAK was observed at the concentrations of 0.195 µg/mL, 1.56 µg/mL, and 0.195 µg/mL against some CNS, MRSA, and MSSA strains, respectively. Similar results were obtained for the combination of moxifloxacin and BAK (data not shown).
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