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J. M. Blondeau, S. Borsos, C. Hesje; Additive Effect of Gatifloxacin and Benzalkonium Chloride (BAK) Against Clinical Isolates of Methicillin-Resistant Staphylococcus Aureus (MRSA). Invest. Ophthalmol. Vis. Sci. 2007;48(13):748.
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MRSA is a challenging therapeutic dilemma given its potential virulence and antimicrobial resistance to many classes of drugs. Previously we showed that gatifloxacin plus BAK had low minimum inhibitory drug concentrations against MRSA strains. In this report, we were interested in determining if MICs were lower for BAK alone versus gatifloxacin and BAK against MRSA strains that were quinolone resistant.
A total of 35 MRSA quinolone resistant isolates were tested. For MIC testing, the Clinical and Laboratory Standards Institute (CLSI) recommended procedure was followed utilizing 105 cfu/ml of MRSA exposed to doubling drug concentrations in Mueller-Hinton broth followed by incubation at 35-37°C in ambient air; the lowest concentration preventing growth was recorded as the MIC. For experiments with BAK, a predetermined concentration was added to the first well of the microtiter plate and serially diluted with drug.
MIC (µg/ml) values were as follows: ciprofloxacin 8-≥128, levofloxacin 4-≥32; moxifloxacin 2-16, gatifloxacin 4-≥8. For gatifloxacin tested with serially diluted BAK, MICs ranged from ≤0.008-0.25 µg/ml. The MIC values for BAK alone ranged from ≤0.1-25 µg/ml which were higher in all instances than for gatifloxacin plus BAK.
Gatifloxacin plus BAK yielded lower MIC values than did either compound alone against the MRSA strains tested. This suggests an additive benefit. As Zymar (gatifloxacin and BAK) contains 50 parts per million of BAK and the average tear volume is approximately 7 µl, the additive benefit of this combination ensures sufficiently high drug concentrations following tear volume dilutions.
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