Longitudinal Trend in Antimicrobial Susceptibility among Haemophilus influenzae: A Single-Centre Study in Japan.

Haemophilus influenzae presents significant concerns regarding antimicrobial resistance. The circumstances surrounding H. influenzae have been changing owing to changes in antimicrobial usage. In the present study, to determine the current situation of H. influenzae, the antimicrobial susceptibility trends were investigated. In total, 21 clinical isolates were analyzed. Antimicrobial susceptibility measured using the broth dilution method was compared with that reported in previous studies at the same hospital. Quinolone low-susceptible isolates were further characterized by multi-locus sequence typing. Upon comparing the susceptibility data in 2022 with those in the past 15 years, the number of β-lactamase nonproducing ampicillin-resistant isolates was decreased. Regarding recent changes, β-lactam-susceptible isolates were found to have gradually increased every year. However, β-lactamase-producing isolates did not decrease. In particular, the ratio of β-lactamase-producing amoxicillin and clavulanic acid-resistant isolates in 2022 was the highest among all the years studied. Moreover, quinolone low-susceptible isolates were still present, suggesting that these isolates could have been indigenized in the community. Furthermore, a β-lactamase-producing amoxicillin and clavulanic acid-resistant isolate was found to have emerged among the quinolone low-susceptibility isolates. At first glance, these findings indicate that antimicrobial resistance has decreased among H. influenzae in clinical settings. However, β-lactamase-producing isolates and quinolone low-susceptible isolates have remained at a constant rate in the community.