Antimicrobial Resistance and Molecular Epidemiology of Haemophilus influenzae Causing Pediatric Acute Otitis Media in Japan

BACKGROUND: Antimicrobial-resistant strains of Haemophilus influenzae are associated with treatment failure and recurrence in acute otitis media (AOM). We aimed to characterize the current antimicrobial susceptibility of H. influenzae isolates from children with AOM, elucidate β-lactam and quinolone resistance mechanisms, and determine the molecular epidemiology of resistant strains. METHODS: We analyzed 182 H. influenzae isolates from the middle ear fluid of children aged ≤3 years with AOM from multiple regions in Japan (February 2022-May 2023). Antimicrobial susceptibility was tested, and β-lactamase genes (blaTEM-1 and blaROB-1) were detected. Sequence analyses of ftsI, encoding PBP3 and quinolone resistance-determining regions were performed to elucidate the resistance mechanisms. Multilocus sequence typing (MLST) was conducted for the ampicillin- and quinolone-low-susceptible strains. RESULTS: Of 182 isolates, 14 (7.7%), 10 (5.5%), 36 (19.8%) and 122 (67.0%) were β-lactamase-producing, β-lactamase-nonproducing ampicillin-resistant (BLNAR), low-BLNAR and β-lactamase-nonproducing ampicillin-susceptible strains, respectively. All 10 BLNAR and 4 β-lactamase-producing isolates exhibited group III PBP3 amino acid substitutions associated with high-level resistance. MLST revealed genetic diversity among BLNAR isolates, whereas 7 (50.0%) β-lactamase-producing isolates belonged to ST103, indicating clonal dissemination. Amino acid substitutions in the quinolone resistance-determining region were detected in six quinolone-low-susceptible strains; these isolates exhibited diverse STs. CONCLUSIONS: The β-lactam susceptibility of H. influenzae isolates causing pediatric AOM exhibited an overall improvement. However, an increasing trend in quinolone-low-susceptible strains was observed. Specific clones represented by ST103 were detected among some of the β-lactamase-producing strains, warranting careful monitoring.