Activities of 13 quinolones by three susceptibility testing methods against a collection of Haemophilus influenzae isolates with different levels of susceptibility to ciprofloxacin: evidence for cross-resistance

The activities of nalidixic acid, ciprofloxacin, norfloxacin, ofloxacin, pefloxacin, flerofloxacin, sparfloxacin, grepafloxacin, gatifloxacin, moxifloxacin, trovafloxacin, levofloxacin and clinafloxacin against a panel of Haemophilus influenzae strains were assessed by three susceptibility testing methods: Etest, agar dilution and the reference broth microdilution method using Haemophilus test medium (HTM) in all cases. The panel included 62 clinical and two reference H. influenzae strains; 32 had decreased susceptibility to ciprofloxacin (MIC > or = 0.12 mg/L) and 30 were susceptible to this antibiotic (MIC < or = 0.06 mg/L). Both Etest and HTM agar dilution results (r = 0.96; 86.61% and 82.1% of MICs within + one log(2), respectively) correlated well with the reference microdilution method. The MIC(90) of ciprofloxacin was 4.0 mg/L (range 0.007-32.0 mg/L). Trovafloxacin activity was similar to that of ciprofloxacin but sparfloxacin, grepafloxacin, ofloxacin, pefloxacin and flerofloxacin activities were higher (with MIC values one log(2) dilution lower than ciprofloxacin). The least active were norfloxacin (MIC(90) 16 mg/L) and nalidixic acid (MIC(90) 128 mg/L). Levofloxacin and moxifloxacin were more active than ciprofloxacin (MIC(90) 2 mg/L); clinafloxacin and gatifloxacin were the most active with an MIC(90) of 0.25 mg/L. Cross-susceptibility among all quinolones was observed (r > 0.9). Resistance to ciprofloxacin was associated with a similar magnitude of activity loss to other new and old quinolones. Ciprofloxacin MIC determination should be sufficient to detect the decreased susceptibility to the whole group of quinolones.