Pathogenicity and Bro gene typing of pediatric lower respiratory tract infections with Moraxella catarrhalis in Southwest Shandong, China.

To investigate the etiology and clinical characteristics of Moraxella catarrhalis infections in the lower respiratory tract among pediatric patients in southwestern Shandong Province, China. This study aims to enhance early identification and diagnostic accuracy for laboratory physicians, while providing evidence to guide clinical diagnosis and treatment of Moraxella catarrhalis-related infections. This retrospective cohort study analyzed pediatric patients with Moraxella catarrhalis lower respiratory tract infections in southwestern Shandong Province, China. Clinical isolates were obtained through standardized sputum/bronchoalveolar lavage collection protocols and subjected to microbiological identification, antimicrobial susceptibility testing, and molecular characterization of β-lactamase production and bro gene variants. Epidemiological patterns and clinical profiles were systematically evaluated using electronic medical record data spanning January 2020 to December 2023. During the 4-year surveillance period (2018-2021), Moraxella catarrhalis was isolated from 848 pediatric cases of lower respiratory tract infections, representing a 7.81% overall detection rate. Age-stratified analysis revealed the highest prevalence in infants aged 28 days to 1 year (9.69%), with significant seasonal variation peaking in the fourth quarter (11.58%, p < 0.05). Monomicrobial infections predominated (79.72%, 676/848), while polymicrobial cases (20.28%, 172/848) predominantly co-occurred with Streptococcus pneumoniae and Haemophilus influenzae. All isolates were confirmed through parallel testing using automated biochemical analyzers and MALDI-TOF mass spectrometry. Antimicrobial susceptibility profiling demonstrated complete susceptibility to ceftazidime, cefepime, and imipenem (100%), with ≥ 95% susceptibility rates to ciprofloxacin (98.2%), levofloxacin (97.6%), ceftriaxone (96.8%), cefuroxime (96.1%), tetracycline (95.4%), and chloramphenicol (95.1%). A concerning temporal escalation in erythromycin resistance was observed (69.73% in 2018 vs. 90.57% in 2021, χ²=41.32, p < 0.001), while ampicillin and clindamycin resistance remained persistently high (> 93% across all years).β-lactamase production was detected in 96.58% (819/848) of isolates, with molecular characterization identifying bro-1 (94.51%, 774/819) and bro-2 (5.49%, 45/819) gene variants. The β-lactamase-negative subgroup (3.42%, 29/848) showed no significant epidemiological clustering. Our surveillance study demonstrates that Moraxella catarrhalis lower respiratory tract infections in southwestern Shandong Province predominantly affect infants aged 28 days to 1 year, with significantly elevated seasonal incidence during the fourth quarter. Notably, we observed a concerning temporal escalation in erythromycin resistance and persistently high resistance rates to ampicillinand clindamycin throughout the 2018-2021 surveillance period. Crucially, β-lactamase hyperproduction particularly BRO-1 gene carriage emerged as the principal resistance mechanism against β-lactams, while maintained susceptibility to expanded-spectrum cephalosporins and carbapenems suggests preserved therapeutic options. These findings underscore the necessity for: Avoidance of macrolides and β-lactam/β-lactamase inhibitor combinations in empirical therapy; Continuous monitoring of BRO gene evolution patterns; Age-specific antimicrobial stewardship programs targeting infant populations.