Characteristics of Haemophilus influenzae Isolates Responsible for Invasive Infections in Poland in 2018–2023

Abstract

Introduction: Haemophilus influenzae is a human-specific Gram-negative bacterium responsible for respiratory tract infection, sepsis, and meningitis. The study aimed to investigate the epidemiology, serotype distribution, and mechanisms of beta-lactam resistance among invasive H. influenzae strains isolated in Poland from 2018 to 2023. Methods: Invasive H. influenzae isolates were received from patients with positive culture results from blood, cerebrospinal fluid (CSF), and pleural fluid. Sample data were obtained from the Polish laboratory surveillance system. For all isolates screening test for beta-lactam resistance was performed and the minimum inhibitory concentrations (MICs) of clinically relevant antibiotics were determined using antibiotic gradient strips. For isolates with inhibition zone P 1U < 12 mm (n = 133), whole genome sequencing (WGS) analysis was performed. Results: Most strains of H. influenzae were isolated from blood (90.7%). Non-typeable Hinf (NTHi) strains were responsible for most invasive disease in all age groups and accounted for 85.7% (342/399) of all cases. Capsulated isolates constituted 14.3%; among them the most common serotype was type f (Hif; 64.9%), followed by serotypes: e (Hie; 19.3%), b (Hib; 14.0%), and d (Hid; 1.8%). Of the 399 isolates collected between 2018 and 2023, 15.8% and 1.0% were resistant to ampicillin and cefotaxime, respectively. Resistance to meropenem and ciprofloxacin using the meningitis breakpoint was detected in 4.3% and 1.8% strains, respectively. All isolates showed susceptibility to chloramphenicol. Resistance to rifampicin characterized 3.8% of isolates tested. The Cefinase test revealed beta-lactamase production in 8.8% of isolates. Conclusions: In our study NTHi predominated among invasive cases across all age groups, especially among elderly patients, similarly to other countries. β-Lactam resistance among studies strains has remained stable over the years. Recently, however, resistance to third-generation cephalosporins has emerged. Continuous surveillance and a rational antibiotic policy are essential to address H. influenzae resistance.