The Role of Soluble Plant Fibres (Non-Starch Polysaccharides, NSP) in the Maintenance of Intestinal Health and Prevention of Diarrhoeal Disease

摘要

It has long been proposed that a high intake of dietary fibre promotes good intestinal health. Work performed previously by our research group suggests that soluble dietary fibre might have a particularly beneficial impact on intestinal health via its ability to inhibit potentially harmful interactions between bacterial pathogens and the gut epithelium. The aims of this thesis were to evaluate soluble plantain NSP for its ability to disrupt the epithelial interactions of diarrhoeal pathogens C. difficile and Enterotoxigenic Escherichia coli (ETEC), as well as other bacterial components implicated in diarrhoeal disease. Work was also performed to characterise the specific inhibitory fraction of plantain NSP, and in addition, to establish the molecular mechanism underlying its inhibitory activity. A range of soluble dietary fibres were shown to significantly inhibit the in vitro epithelial adhesion of C. difficile and ETEC, but out of all the fibres tested, soluble plantain NSP exhibited the highest efficacy. Plantain NSP also significantly inhibited the epithelial adhesion of eleven C. difficile clinical isolates, irrespective of their toxin expression or ribotype status. Furthermore, plantain NSP blocked the epithelial interactions of five purified C. difficile spore preparations. In addition to its anti-adhesive effects, soluble plantain NSP significantly down-regulated the pro-inflammatory, cytotoxicity and apoptotic response induced by C. difficile and its toxins. Similar effects were also found with respect to mucosally-associated E. coli isolated from ulcerative colitis (UC) patients, as well as bacterial components such as flagellin and LPS. Results demonstrated that the inhibitory activity of plantain NSP was mediated by its acidic polysaccharide fraction, which is mainly composed of pectic material. In addition, it was shown that soluble plantain disrupted bacterial-epithelial interactions via an interaction with the intestinal epithelium. Whilst plantain NSP induced increased cellular chloride secretion, this mechanism was not responsible for inhibitory activity. It was also hypothesised that plantain NSP might mimic intestinal MUC2 glycans by interacting with cell-surface galectin-3, with consequent nuclear localisation of β-catenin and down-regulation of inflammatory cytokines. Whilst plantain NSP was shown to induce activation of β-catenin, the knockdown of surface galectin-3 expression had no effect on inhibitory activity. Thus, the specific mechanism underlying the inhibitory activity of plantain NSP requires further investigation. This work supports the hypothesis that soluble plantain fibre can inhibit harmful interactions between bacteria and the human intestinal epithelium. Indeed, these studies provide convincing evidence to suggest that soluble plantain fibre, acting as a ‘contrabiotic’, could be developed as a potential prophylaxis or treatment against C. difficile and ETEC, which represent the main cause of antibiotic-associated diarrhoea and traveller’s diarrhoea, respectively. In addition, dietary supplementation with soluble plantain NSP may also confer a therapeutic benefit in inflammatory bowel disease (IBD).