Identification and characterization of factors contributing to the pathogenicity of Moraxella catarrhalis
Buskirk, Sean William
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Moraxella catarrhalis is a human-specific pathogen and causative agent of otitis media in children and respiratory tract infections in adults. During the course of pathogenesis, M. catarrhalis is able to colonize and persist in the human host through processes such as adherence to the mucosal epithelium and resistance to complement-mediated killing. The first part of this dissertation describes the identification and characterization of the protein responsible for cardiolipin synthesis in M. catarrhalis (MclS) and the contribution of cardiolipin to bacterial adherence to human epithelial cells. To study the role of MclS, we constructed an mclS insertion mutant strain that lacks expression of MclS and an mclS point mutant strain that expresses an enzymatically-inactive version of MclS. Both mutant strains are unable to synthesize cardiolipin and are defective in their adherence to human epithelial cells in vitro, thereby suggesting that MclS mediates adherence in an indirect manner. We hypothesize that cardiolipin contributes to adherence through its interaction with surface-displayed adhesins. Examination of outer membrane vesicles revealed that cardiolipin is present in the outer membrane of M. catarrhalis. However, five major proteinaceous adhesins (UspA1, Hag, McaP, OMP CD, MhaB1/MhaB) were all expressed and displayed on the surface of the cardiolipin-deficient strains. Conversely, expression of lipooligosaccharide, which has previously been implicated in adherence of M. catarrhalis to epithelial cells, may depend on the presence of cardiolipin. The second part of this dissertation describes a series of experiments designed to identify factors that contribute to complement resistance of M. catarrhalis. Several factors are currently known to contribute to complement resistance including UspA2 which mediates the majority of resistance exhibited by M. catarrhalis. Using experimental evolution, we demonstrate that an M. catarrhalis strain lacking expression of UspA2 is able to develop complement resistance through repeated exposure to human serum. We intend to identify the mutations that confer complement resistance in the evolved lineages using whole-genome sequencing. Ultimately, we hope to gain a better understanding of the factors that impact complement resistance in M. catarrhalis.