Elucidating the role of the mucus-associated microbiota and mucin glycosylation in inflammatory bowel disease

The human gastrointestinal (GI) tract is host to a dynamic community of 1013-1014 bacteria, which mainly reside in the colonic lumen and outer mucus layer covering the GI tract. Mucins are decorated in a diverse array of O-glycans, providing nutrients and attachment sites for microbes. Dysbiosis of the microbiota, and alterations in mucin glycosylation have been associated with Inflammatory Bowel Disease (IBD). However, the causal relationship between these two factors remains unclear. Here, we employed a multidisciplinary approach to address this relationship, and the molecular mechanisms mediating these changes. Mucosal lavages and biopsies obtained from the sigmoid and ascending colon of patients were used to assess alterations in the mucosal microbiota and the glycosylation of associated mucus in ulcerative colitis (UC) patients. Secondly, in vitro growth assays and gnotobiotic mouse experiments were performed to investigate the reciprocal role of mucin-degrading bacteria in the modulation of mucin O-glycosylation. These analyses highlighted inter-patient variability, but a similar microbial composition between colonic sites. In contrast, mucin glycosylation and the expression of glycosyltransferases was regio-specific. In UC, changes in the abundances of bacterial groups, including a decrease in the A. muciniphila to R. gnavus ratio were apparent. UC mucins displayed a decrease in fucosylation, increase in sialylation, and a decrease in many complex glycan structures found in abundance in controls. In vitro growth assays suggested that UC-like mucin glycosylation impaired A. muciniphila growth, whilst R. gnavus remained unaffected, potentially explaining changes in these species in UC. Furthermore, gnotobiotic mouse experiments showed that A. muciniphila and R. gnavus were able to remodel mucin glycosylation. Our findings suggest a multifactorial dysregulation at the epithelial interface in IBD, where mucus-associated microbiota and mucin glycosylation are interdependent. It is likely that an initial disruption in either of these components drives alterations in other mucosal constituents, propagating disease and exacerbating inflammation

Use of Atomic Force Microscopy to Study the Multi-Modular Interaction of Bacterial Adhesins to Mucins

The mucus layer covering the gastrointestinal (GI) epithelium is critical in selecting and maintaining homeostatic interactions with our gut bacteria. However, the molecular details of these interactions are not well understood. Here, we provide mechanistic insights into the adhesion properties of the canonical mucus-binding protein (MUB), a large multi-repeat cell–surface adhesin found in Lactobacillus inhabiting the GI tract. We used atomic force microscopy to unravel the mechanism driving MUB-mediated adhesion to mucins. Using single-molecule force spectroscopy we showed that MUB displayed remarkable adhesive properties favouring a nanospring-like adhesion model between MUB and mucin mediated by unfolding of the multiple repeats constituting the adhesin. We obtained direct evidence for MUB self-interaction; MUB–MUB followed a similar binding pattern, confirming that MUB modular structure mediated such mechanism. This was in marked contrast with the mucin adhesion behaviour presented by Galectin-3 (Gal-3), a mammalian lectin characterised by a single carbohydrate binding domain (CRD). The binding mechanisms reported here perfectly match the particular structural organization of MUB, which maximizes interactions with the mucin glycan receptors through its long and linear multi-repeat structure, potentiating the retention of bacteria within the outer mucus layer