Virulence and Host Adaptation Mechanisms of Non-typeable Haemophilus influenzae

Non-typeable Haemophilus influenzae (NTHi) resides in the human respiratory tract as a commensal but is also a major cause of mucosal infections and exacerbations of chronic obstructive pulmonary disease (COPD).The increasing prevalence of NTHi infections, together with the emergence of antibioticresistant strains, emphasises the need for a deeper understanding of NTHi virulence strategies and hostpathogen interactions.This thesis aimed to elucidate adaptive and immune-evasive mechanisms by NTHi during colonisation and infection of the human respiratory tract.In Paper I, we investigated how an airway-simulated culturing medium and temperature influence NTHi pathogenic behaviour.Global transcriptional analyses revealed that growth in airway-simulated medium induced metabolic adaptations resembling those observed in vivo, while lower temperatures enhanced bacterial adherence but reduced serum resistance possibly through coordinated regulation of adhesins and outer membrane proteins.Papers II and III focused on complement evasion mediated by the major outer membrane protein P5.We identified P5 as the bacterial receptor for binding to complement inhibitors C4b-binding protein (C4BP) and vitronectin, demonstrating its contribution to serum resistance and immune evasion.Surface exposed loop 2 of P5 was identified as the most likely binding site for both C4BP and vitronectin, highlighting the role of P5 in immune evasion.In Paper IV, we demonstrated that the C-terminal domain of P5 binds peptidoglycan and contributes to bacterial membrane stability.Deletion of this domain altered outer membrane and periplasmic protein composition and impaired hostinteraction phenotypes, including adherence and serum survival.Finally, Paper V identified the transcriptional regulator FabR as a potential repressor of P5 expression and a contributor to NTHi adherence, immune evasion, metabolic regulation, and in vivo virulence.In summary, this thesis demonstrates that NTHi virulence is driven by coordinated adaptations to the host environment, involving metabolic regulation, temperature-responsive gene expression and multifunctional outer membrane proteins.These findings provide new insights into NTHi pathogenesis and identify potential targets for future therapeutic strategies.