Anti-pneumococcal activity of gatifloxacin compared with other quinolone and non-quinolone agents

An agar dilution MIC method was used to test the activity of gatifloxacin, a new broad-spectrum fluoroquinolone, compared with ciprofloxacin, levofloxacin, sparfloxacin, trovafloxacin, amoxycillin, cefuroxime, ceftriaxone and clarithromycin against 71 penicillin-susceptible, 81 penicillin-intermediate and 55 penicillin-resistant pneumococci. Quinolone activity was unaffected by penicillin susceptibility, with MIC50/MIC(90)s (mg/L) of 0.25/0.5 for gatifloxacin; 1/2 for ciprofloxacin; 1/2 for levofloxacin; 0.25/0.5 far sparfloxacin; 0.125/0.25 for trovafloxacin. beta-Lactam and clarithromycin MICs rose with those of penicillin G; MIC50/MIC90 values (mg/L) for penicillin-susceptible, -intermediate and -resistant strains were: 0.03/0.06, 0.25/1, 2/4 for penicillin G; 0.03/0.03, 0.125/1, 2/4 for amoxycillin; 0.03/0.125, 0.5/4, 8/16 for cefuroxime; 0.03/0.03, 0.25/0.5, 2/4 for ceftriaxone; 0.03/0.06, 0.03/>64, 1/>64 for clarithromycin. Time-kill testing of four penicillin-susceptible, four -intermediate and four -resistant strains showed that levofloxacin at the MIG, gatifloxacin and sparfloxacin at 2 x MIC, and trovafloxacin and ciprofloxacin at 4 x MIC, were bactericidal (99.9% killing) for all strains after 12 h and 24 h. By contrast, amoxycillin, cefuroxime and ceftriaxone showed bactericidal activity after 24 h against all strains at 4, 8 and 4 x MIC, respectively. Against ten organisms with clarithromycin MICs of 0.03-4.0 mg/L, clarithromycin was bactericidal against seven strains at 8 x MIC after 24 h. Quinolones showed mote rapid killing at lower concentrations and earlier time periods than did beta-lactams and clarithromycin.