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

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. 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. 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