β-Lactamase-non-producing ampicillin-resistant Haemophilus influenzae is acquiring multidrug resistance.

BACKGROUND: Haemophilus influenzae strains with reduced susceptibilities to antimicrobial agents have emerged in Japan. Here, we aimed to investigate H. influenzae non-susceptibility to β-lactams and non-β-lactams.

METHODS: A total of 260 H. influenzae isolates from patients in 2013-2016 were analysed. Antimicrobial susceptibilities were assessed by determining the minimum inhibitory concentration. Additionally, isolates with reduced susceptibility were analysed by both genetic and statistical methods.

RESULTS: β-Lactamase-non-producing ampicillin-resistant H. influenzae (BLNAR) strains increased significantly and accounted for more than 50% of all isolates from 2014. Additionally, the proportion of quinolone-low-susceptibility isolates increased significantly (P<0.05). Among these, three quinolone-non-susceptible isolates showed minimum inhibitory concentrations higher than the susceptibility breakpoint of levofloxacin. Moreover, one of the three isolates showing multidrug resistance was resistant to macrolides, β-lactams, and quinolones. Low susceptibilities to non-β-lactams were significantly associated with BLNAR.

CONCLUSIONS: The present study indicates that BLNAR strains are increasing and tend to show multidrug resistance. Additionally, multidrug-resistant H. influenzae (MDRHI) has emerged. To prevent the further spread of MDRHI, the proportions of BLNAR strains should be evaluated.