In the post-vaccine era, the nontypeable (i.e. nonencapsulated) Haemophilus influenzae (NTHi) revealed as an opportunistic pathogen causing and exacerbating multiple upper and lower respiratory tract diseases. Worryingly, several studies in various post-vaccine populations have observed a steadily increase of NTHi invasive incidence rates and antibiotic resistance. In this thesis work, we provide evidence indicating that altered penicillin-binding protein 3 (PBP3), slowed drug influx and direct efflux regulation contribute to the development of imipenem hetero-resistance in NTHi. We then established that the enhancement of imipenem-heteroresistant NTHi susceptibility to imipenem under heat stress conditions depends largely on the expression levels of PBP1b, AcrAB-TolC, and OmpP2, indicating again the role of the same pathways. Finally, we characterized the mechanisms of resistance to fluoroquinolones and macrolides in H. influenzae; assessed the extent of the AcrAB-TolC-mediated imipenem resistance; and defined a core genome multilocus sequence typing (cgMLST) scheme for H. influenzae by using whole-genome sequencing.