Emergence of Haemophilus influenzae with low susceptibility to quinolones and persistence in tosufloxacin treatment

Abstract Background In Japan, usage of non-β-lactam agents has increased due to the prevalence of β-lactam resistant pathogens. This study aimed to clarify the recent trend of antimicrobial susceptibility and molecular epidemiological features in Haemophilus influenzae . Methods Fifty-seven H. influenzae isolated from a Japanese teaching hospital in 2017 were characterized, and the data were compared with those of a previous study. The MICs were determined using the broth dilution methods Genetic backgrounds were compared by Multilocus sequence typing. The bactericidal activity of tosufloxacin at, or near, the therapeutic Cmax was determined in vitro with susceptible and levofloxacin low-susceptible isolates by time kill assay. Results of susceptibility tests showed that more than 90% of isolates were susceptible to cephalosporins and carbapenems, whereas ampicillin-susceptible and clarithromycin-susceptible isolates decreased. With regards to quinolones, low-susceptible isolates were noted in 2017, although all isolates were judged as susceptible. All low-susceptible isolates had an amino acid substitution in GyrA, and two isolates had an additional substitution in ParC. These isolates had different genetic background. Furthermore, the time-kill kinetic assay using the Cmax of tosufloxacin indicated that the low-susceptible isolates could persist for at least 8 hours. Conclusions This study revealed that H. influenzae demonstrated multidrug low-susceptibility in recent years. The low-susceptible isolates had genetic diversity, meaning that resistance occurred independently.