Antimicrobial activity and accumulation of moxifloxacin in quinolone-susceptible bacteria

The antibacterial activity of moxifloxacin, compared with that of ciprofloxacin, was determined for five strains of Staphylococcus aureus, including one NorA-overproducing strain, two quinolone-susceptible strains of Streptococcus pneumoniae, four quinolone-susceptible strains of Haemophilus influenzae, and one strain each of quinolone-susceptible Escherichia coli, Pseudomonas aeruginosa and Moraxella catarrhalis. In addition, the accumulation of moxifloxacin and ciprofloxacin by the NCTC type strain of S. pneumoniae, H. influenzae, S. aureus, E. coli and P. aeruginosa was determined by a fluorescence method. For all strains, moxifloxacin accumulated to a lower concentration than ciprofloxacin. The concentrations of moxifloxacin accumulated ranged from 12 to 44 ng/mg dry cells. The lowest concentration was accumulated by S. pneumoniae NCTC 7465 and the highest concentration by S. aureus NCTC 8532. Increased expression of norA in S. aureus had no effect on the accumulation of moxifloxacin. Despite differences in the concentration of moxifloxacin accumulated by the different species, there was little difference between the MICs of this agent for each strain (0.06-0.5 mg/L), suggesting that the concentration accumulated by wild-type bacteria has little effect on the MIC.