Graduate Thesis Or Dissertation
 

Disease models and infectious phenotypes of Mycobacterium avium subspecies paratuberculosis

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  • Mycobacterium avium subspecies paratuberculosis (MAP) is the causative agent of Johne's disease, a chronic inflammatory bowel disease that affects ruminant populations worldwide. The characteristic stages of the disease make diagnosis difficult, resulting in silent transmission among animals in a herd for years before proper detection of the infection. The extensive prevalence of Johne's disease has driven a continuous effort to more readily understand the pathogenesis of the bacterium and to develop more effective preventative measures to curb the spread of the disease within herds. In this dissertation, we aim to create a more effective model for studying MAP infection within the intestinal mucosa, to utilize the milk-induced virulence phenotype to study how opsonization affects cell infection, and to study the metabolic interaction between MAP and the host phagocyte during infection. We describe a novel in vitro cell culture passage model which indicates that MAP changes during passage between bovine epithelial cells and macrophages, developing a more pro-inflammatory phenotype. We show that the inflammatory MAP phenotype not only increases gene expression of lipid metabolism- and modification-related genes, but that it is also composed of a set of lipids that are unique to the phenotype. Ultimately, we were able to identify these inflammatory-related transcripts in naturally MAP-infected bovine tissues, thus validating our model and indicating that the changing MAP phenotype may be a contributing factor in driving the development of inflammation within MAP infected animals. By using a different infectious phenotype that develops after MAP exposure to milk, a reservoir and transmission source of the bacterium, we demonstrate that opsonization of MAP results in more efficient translocation across an epithelial monolayer. Upon infection, we determine that macrophages more readily kill opsonized MAP in a rapid and specific manner. Furthermore, we begin to characterize one of the highest upregulated genes in this milk-induced phenotype, MAP1203, and its interaction with the intestinal epithelium. We establish that the putative cell wall associated protein is involved in both binding to and invasion of bovine MDBK epithelial cells when over-expressed in MAP during infection. Together, these data indicate the importance of the infectious phenotype developed after milk exposure and its role in the pathogenesis and transmission of Johne's disease. Finally, we utilize Acanthamoeba castellanii (amoeba) as a phagocytic host and describe the influence that MAP infection has on the metabolic activity of the cell. We detail how MAP stimulates the metabolism of the amoeba and how that stimulation directly mirrors the pattern of survival and the intracellular burden of MAP over the course of infection. We identify bacterial mutants that result in excessive or deficient stimulation of the metabolic activity within host cell and by utilizing phenotype arrays, we illustrate that amoeba change the use of specific carbon sources based on the MAP strain used for infection. These data aid in beginning to understand the bacterial mechanisms that drive the metabolic interactions between MAP and the phagocytic host. Our results describe novel model systems for studying Johne's disease and further our understanding about the host-pathogen interactions that occur within the intestinal mucosa of infected cattle. We show that the shifting phenotypes of MAP may be important contributing factors for detection, diagnosis, and in driving the progression of Johne's disease. These findings offer new methods and the identification of new bacterial phenotype targets that could be used as the basis of developing more efficacious strategies in detecting and preventing Johne's disease.
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