Susceptibility of twenty penicillin-susceptible and -resistant pneumococci to levofloxacin, ciprofloxacin, ofloxacin, erythromycin, azithromycin, and clarithromycin by MIC and time-kill

Abstract

This study uses MIC and time-kill methodology to examine the antipneumococcal activity of levofloxacin, ciprofloxacin, ofloxacin, erythromycin, azithromycin, and clarithromycin against 20 pneumococci. Ten strains had levofloxacin MICs of 1.0 μg/ml, and ten levofloxacin MICs of 2.0 μg/ml. Five strains in each group were macrolide susceptible, and five were macrolide resistant. MICs for ciprofloxacin and ofloxacin ranged from 0.5 to 4.0 μg/ml and 1.0 to 8.0 μg/ml, respectively. MICs of erythromycin, azithromycin, and clarithromycin were similar for macrolide susceptible strains, ranging between 0.004 to 0.06 μg/ml, and were ≥128.0 μg/ml for macrolide resistant strains. The three quinolones were bactericidal (99.9% killing) for all macrolide-susceptible strains at 2 × MIC at 24 h. The three quinolones yielded 99% killing of all strains after 12 h at 2 × MIC, and 90% killing of all strains after 6 h at the MIC of levofloxacin and ciprofloxacin and 2 × MIC for ofloxacin. Levofloxacin yielded 90% killing of all strains after 4 h at 2 × MIC and ofloxacin at 4 × MIC. For macrolide-susceptible strains, erythromycin and clarithromycin were bactericidal for 9 of 10 strains after 24 h at 4 × and 2 × MIC, respectively, and azithromycin was bactericidal after 24 h at 2 × MIC for 8 of 10 strains. All three macrolides were bactericidal after 12 h only, while 90% killing occurred in 9 of 10 strains at 8 × MIC after 6 h. Quinolone kill kinetics were similar for the 10 macrolide-resistant strains. For macrolide-resistant strains, at 64 to 128.0 μg/ml, virtually no decrease in count was seen, with no bactericidal effect.