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Christine K. Hesje, Christine M. Sanfilippo, Wolfgang Haas, Timothy W. Morris; The Mutant Prevention Concentration (MPC) and Mutation Frequency (MF) of Ocular Staphylococcus aureus Isolates Selected with Besifloxacin and Comparator Ophthalmic Fluoroquinolone Agents. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1496.
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The MPC is a measurement of in vitro susceptibility defined as the lowest antimicrobial concentration at which no resistant mutants are recovered. Here we investigate the proportion of resistant mutants recovered for fluoroquinolone- (FQ) and methicillin- susceptible S. aureus ocular isolates by in vitro selection with besifloxacin and comparator FQ agents.
Minimum inhibitory concentration (MIC) and MPC values were determined for 10 isolates (8 clinical and 2 quality control strains). MIC values were determined using the broth microdilution method according to Clinical and Laboratory Standards Institute guidelines, while MPC values were determined by plating 109 - 1010 colony-forming units (CFUs) on agar containing serially-diluted concentrations of besifloxacin, moxifloxacin, gatifloxacin, and ciprofloxacin. The MF of each isolate against all FQ agents was determined by counting CFUs on 4-fold FQ MIC agar plates.
The MIC, MPC, and MF results for all four agents are listed in the table below.
The MIC90 and MPC90 values show that the potencies of the four FQ agents tested were consistently as follows: besifloxacin > moxifloxacin > gatifloxacin > ciprofloxacin. In addition, the selection of S. aureus mutants by the newest FQ agent besifloxacin occurs at a very low frequency; when compared to other agents, the MF of S. aureus selected with besifloxacin was substantially lower than that observed with ciprofloxacin but similar to the MF of mutants selected by moxifloxacin and gatifloxacin. Due to the increasing prevalence of FQ resistance in S. aureus ocular infections, it will be important to investigate whether similar MPC and MF trends are observed among such isolates in future studies.
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