Comparison of the bactericidal effects of quinolones against low-susceptible Haemophilus influenzae.

The increasing incidence of Haemophilus influenzae with decreased susceptibility to quinolones (quinolone low-susceptible H. influenzae) in Japan has raised concerns about therapeutic failure. Thus, assessment of effective antimicrobial agents is necessary to establish an effective therapeutic strategy against resulting infections. In this study, in vitro bactericidal effects of quinolones on low-susceptible H. influenzae strains were evaluated using time-kill curve analysis. For tosufloxacin, log reduction values for low-susceptible strains were significantly lower than those for susceptible strains at both Cmax and 1/2 Cmax. Conversely, although the log reduction values were lower for susceptible strains, the Cmax of levofloxacin and ß-lactams (amoxicillin and cefditoren) indicated bactericidal effects. In addition, higher concentrations of tosufloxacin at 2×Cmax and 4×Cmax had bactericidal effects on not only susceptible but also low-susceptible strains. These data strongly suggest that we should consider the presence of low-susceptible strains and reconsider the appropriate dosage of tosufloxacin for treatment, especially for paediatric patients.