Levofloxacin selects fluoroquinolone-resistant methicillin-resistant Staphylococcus aureus less frequently than ciprofloxacin

Seventeen methicillin-resistant Staphylococcus aureus (MRSA) with unique genotypes were examined to determine if resistance occurs more frequently with ciprofloxacin or levofloxacin. The mean single-step resistance rate to 4 x MIC of ciprofloxacin was 1.05 x 10(-5) (range < 4.82 x 10(-11) to 5.06 x 10(-5)), and that to levofloxacin was 4.03 x 10(-6) (range < 3.57 x 10(-13) to 4.10 x 10(-5)) (P < 0.05). When serially passaged, the mean MICs of ciprofloxacin and levofloxacin increased 3.0 +/- 1.5-fold and 1.8 +/- l.4-fold, respectively (P < 0.05). Only four strains became resistant to levofloxacin, but eight became resistant to ciprofloxacin.