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A mucus hydrogel covers the intestinal epithelium, protecting the host from passing food and resident gut microbiota. The mucus consists of a highly organized glycoprotein network, mainly produced by goblet cells. In the distal colon, where microbial abundance is highest, goblet cells continuously secrete mucus, which “grows” by expansion, forming a gradient from an inner nearly sterile layer to an outer loose layer where bacteria reside. This pushes microbes away from the host epithelium, thereby reducing the risk of infection and inflammation.

The gut microbiota is predominantly composed of bacteria and is strongly influenced by diet. Individuals in industrialized societies exhibit reduced microbial diversity compared to those in non-industrialized societies – a difference partly attributed to the Western-style diet (WSD), which is low in dietary fiber and high in simple sugars and saturated fat. In mice fed a WSD, microbiota diversity is reduced, and the mucus barrier is weakened, as seen by a slower mucus growth rate and increased bacterial penetration, which raises the risk for harmful microbial interactions. Similar microbial and mucus alterations are observed in patients with inflammatory bowel disease (IBD). While mucus properties are microbiota-dependent, the underlying regulatory mechanisms are still largely elusive. This thesis aims to clarify these mechanisms by studying the colonic mucus barrier in a WSD environment, with both human and mouse microbiota. Moreover, the effects of structures previously untested for their mucus-influencing properties during WSD feeding are evaluated in mice.

In study 1, the impact of diet-induced changes in the human microbiota on the colonic mucus barrier was studied by transplanting fecal samples into microbiota-depleted mice. Healthy participants increased their fiber intake over three months, and their microbiota was collected before and after the intervention. When transplanted into mice, only the high-fiber-derived microbiota maintained mucus growth under WSD feeding and reduced the pathogen load during intestinal infection. The bacterial taxon Blautia was enriched in the high-fiber group, and Blautia coccoides emerged as a key regulator of mucus integrity through the production of the short-chain fatty acids (SCFAs) acetate and propionate. These metabolites were shown to stimulate mucus growth via the free-fatty acid receptor 2 (Ffar2), revealing a previously unrecognized mechanism by which microbial metabolites directly impact mucus integrity.

In studies 2 and 3, the mucus-influencing effects of bovine milk-derived casein glycomacropeptide (CGMP) and human milk oligosaccharide (HMO) structures were examined in WSD-fed mice. CGMP is a glycosylated protein found in cheese whey, while HMOs are breast milk components with over 200 known structures. Both GCMP and HMOs share structural similarities with mucin glycans and may act as decoy substrates for bacterial degradation under low-fiber conditions. In study 2, CGMP structures improved mucus growth rate, where specifically, a highly sialylated CGMP (HSA) increased propionate levels and the relative abundance of Bifidobacteria, a bacterium previously linked to mucus maintenance. In study 3, specific HMO structures enhanced mucus integrity and improved mucus penetrability in a mouse model for IBD. HMO-dependent mucus modulation could be linked to changes in bacterial composition, increased SCFA levels and glycan-targeting enzyme activities. These findings emphasize the structure-specific effects of CGMPs and HMOs and their distinct modulation of microbiota–mucus interactions.

In summary, this thesis reveals new insights into how microbial metabolites regulate the mucus barrier in the Western gut and highlights novel strategies to be exploited for treating mucus-associated disorders such as IBD.