Nontypeable Haemophilus influenzae High Molecular Weight Adhesins: Molecular Epidemiology, Evolution, and Within-Host Population Dynamics.

Abstract

Nontypeable Haemophilus influenzae (NTHi), a Gram-negative bacterium that commonly resides within the human pharynx as a commensal, is also capable of causing localized infections of the respiratory tract and invasive disease. Among children, NTHi is a leading cause of acute otitis media (AOM) which is a significant cause of childhood morbidity and the most common reason for prescribing antibiotics in the US. In adults, NTHi is often associated with acute exacerbations of chronic obstructive disease (COPD), the fourth leading cause of death worldwide. Because NTHi is human restricted its long term survival is dependent upon its ability to successfully colonize new hosts. Adherence to host epithelium, mediated by bacterial adhesins, is proposed to be one of the first steps in bacterial colonization and since disease-causing NTHi strains originate from strains that colonize the pharynges, adherence also marks one of the first steps in NTHi pathogenesis. NTHi encode several adhesins, including the high molecular weight (HMW) adhesins that mediate attachment to the respiratory epithelium where they interact with the host immune system, eliciting a strong humoral response. hmwA, which encodes the HMW adhesin, displays marked amino acid diversity and also demonstrated phase variation mediated by 7-base pair tandem repeats located within the hmwA promoter region. Thus, to gain further insight into the role of HMW adhesins during colonization and disease I: (1) described a bexB-based molecular typing method that differentiates typeable from true nontypeable H. influenzae, (2) sequenced the hmwA core binding domains from a collection of 170 geographically distributed commensal and AOM isolates, demonstrating that they form four distinct phylogenetic clusters, (3) found evidence of positive selection operating on the hmwA region that encodes the core binding domain, and (4) demonstrated, using a mathematical model, that the occurrence of large, yet rare, phase variable deletion events allows for the stable maintenance of a small population of adherent NTHi cells during colonization in spite of HMW-specific antibody mediated immunity. This work has direct implications for ongoing efforts aimed at developing an effective NTHi vaccine and, at a more basic level, advances our understanding of host-pathogen, and importantly, host-commensal interactions.