Microbial etiology of Inflammatory Bowel Disease: Microbial diversity and the role of Escherichia coli
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Inflammatory bowel disease (IBD), comprises Crohn’s disease (CD) and ulcerative colitis (UC), and is a chronic relapsing inflammation of gastrointestinal tract without any known cause or cure. Currently, it is accepted that IBD is a result of a dysfunctional immune response to commensal bacteria in a genetically susceptible host, and that environmental factors can trigger the onset or reactivation of the disease. This thesis considers the possibility of a specific pathogenic agent as well as an imbalance in the composition of the normal microflora in the pathogenesis of IBD. Gut biopsy tissues were taken from a population-based case-control tissue bank held at the University of Manitoba. Automated ribosomal intergenic spacer analysis (ARISA) and terminal restriction fragment length polymorphisms (T-RFLP) were employed to assess the diversity of gut microbiota. The phylogenetic, virulence and biochemical characteristics of Escherichia coli isolated from IBD biopsies were examined using multi-locus sequence typing (MLST), DNA microarray technology and API 20E system. Utilizing ARISA and T-RFLP, a remarkable increase in the order of unclassified Clostridia was detected in inflamed tissues, particularly in CD patients (P < 0.05). Moreover, species richness and diversity were the highest in non-inflamed IBD biopsies. Culture-based quantification detected a significantly higher number of E. coli in IBD tissues (P < 0.05). Phylogenetic analysis revealed the tendency of E. coli isolated from IBD patients to be grouped into separate clonal clusters based on their allelic profiles (P = 0.02). A link was detected between uropathogenic E. coli (UPEC) CFT073 and strains isolated from IBD, with regards to gene distribution and virulence, using microarray technology. Amino acid substitutions N91S and S99N in FimH, the adhesive subunit of E. coli type I fimbria, were significantly associated to IBD (P < 0.05). This study demonstrated an increase in the microbial diversity of non-inflamed IBD tissues and suggested a recruitment phase of bacterial adherence and colonization, before the inflammation sets in. Furthermore, E. coli isolated from IBD tissues were distinct from commensal strains in both clonal and virulence characteristics and shared remarkable traits with extraintestinal pathogenic E. coli. Features involved in bacterial adhesion to epithelial cells may hold the key to E. coli pathogenesis in IBD.