Comparative in vitro activity of three fluoroquinolones against clinical isolates by E test

The in vitro susceptibility of levofloxacin, ciprofloxacin and moxifloxacin against several gram-positive and gram-negative clinical isolates was tested by E test. We found that the MIC(50) and MIC(90) values against all members of the Enterobacteriaceae family except Serratia were <0.5 mg/l for ciprofloxacin and levofloxacin (MIC range 0.006-32 mg/l) based on the in vitro susceptibility data. The susceptibility rates for ciprofloxacin and levofloxacin were more than 85% for Escherichia coli, citrobacter, enterobacter cloacae, enterobacter aerogenes and Klebsiella pneumoniae, although Serratia and Acinetobacter exhibited more or less similar susceptibility rates (about 80%). Pseudomonas aeruginosa demonstrated significant resistance to fluoroquinolones (MIC(90) >32 mg/l) and decreased bactericidal rates (<65%) to levofloxacin and ciprofloxacin. Respiratory pathogens such as Streptococcus pneumoniae and Haemophilus influenzae were highly susceptible (100%) to levofloxacin and moxifloxacin. The ineffectiveness of fluoroquinolones for treating coagulase-positive Staphylococcus aureus was demonstrated by poor in vitro susceptibility rates with levofloxacin (52%) and moxifloxacin (57%). Coagulase-negative staphylococci demonstrated significantly decreased bactericidal rates to levofloxacin (21%), while the in vitro susceptibility to moxifloxacin was higher (66%) than that to levofloxacin. We propose that the beneficial effect of inclusion of any of these three fluoroquinolones in treating Enterococcus infections is marginal, as demonstrated by significantly reduced susceptibility rates (<32%). These data demonstrate the utility of fluoroquinoles to treat several gram-negative bacterial infections (with the exception of Acinetobacter and P. aeruginosa), as well as S. pneumoniae and H. influenzae.