Antimicrobial resistance characteristics and associated molecular mechanisms of clinically isolated Haemophilus influenzae from the lower respiratory tract in Chongqing, China

Background Global systematic research on Haemophilus influenzae drug resistance profiles and mechanisms is scarce. Many studies are constrained by single-center designs and restricted sample numbers, hindering the proper monitoring of drug resistance trends and the analysis of drug resistance mec e antimicrobial resistance mechanisms. Results Ampicillin had the highest resistance rate (76.2%), followed by azithromycin (38.2%) and amoxicillin/clavulanic acid (23.8%); tetracycline (6.2%) and levofloxacin (0.6%) had low resistance rates. No resistance to ceftazidime, cefotaxime, or meropenem was detected. The β-lactamase positive rate was 67.2%. The TEM gene detection rate was 67.2%, frequently coexisting with mef(A) , msr(D) , and tet(B) to form the local core resistome. Resistance mechanisms: ampicillin resistance (β-lactamase + ftsI /PBP3 mutations); macrolide resistance ( mef(A) / msr(D) efflux pumps); tetracycline resistance ( tet(B) ); levofloxacin resistance (multiple ParC amino acid substitutions). Conclusion Chongqing H. influenzae isolates had high ampicillin resistance, while the resistance to azithromycin and amoxicillin/clavulanate cannot be ignored. The underlying resistance mechanisms were marked by the co-occurrence of macrolide and tetracycline-resistance genes on a TEM gene background. Continuous local surveillance is warranted to optimize empirical antimicrobial therapy and anticipate emerging resistance risks.