Using the acute dextran sulfate sodium (DSS)induced colitis model, studies have demonstrated that intestinal inflammation is accompanied by major changes in the composition of the intestinal microbiota. Only little is known about the microbial changes and more importantly their functional impact in the chronic DSS colitis model. We used a refined model of chronic DSS-induced colitis that reflects typical symptoms of the human disease without detrimental weight loss usually observed in DSS models. We sampled cecum and colon content as well as colon mucus from healthy and diseased mouse cohorts (n = 12) and applied 16S rRNA gene sequencing and metaproteomics. An increase of Prevotella sp. in both colon content and mucus was observed. Functional differences were observed between sample types demonstrating the importance of separately sampling lumen content and mucus. The abundance of Desulfovibrio, a sulfate-reducing bacterium, was positively associated with the carbon metabolism. Lachnoclostridium was positively correlated to both vitamin B6 and tryptophan metabolism. In summary, functional changes in the distal colon caused by DSS treatment were more pronounced in the mucusassociated microbiota than in the microbiota present in the distal colon content.

Inflammatory bowel diseases (IBD), such as Crohn’s disease and ulcerative colitis, are multifactorial inflammatory disorders of the gastrointestinal tract, characterised by abdominal cramps, bloody diarrhoea, and anaemia. Standard therapies, including corticosteroids or biologicals, often induce severe side effects, or patients may develop resistance to those therapies. Thus, new therapeutic options for IBD are urgently needed. This study investigates the therapeutic efficacy and safety of two plant-derived ligands of the aryl hydrocarbon receptor (AhR), quercetin (Q), and indol-3-carbinol (I3C), using a translationally relevant mouse model of IBD. Q and I3C are administered by gavage to C57BL/6 wild-type or C57BL/6 Ahr-/- mice suffering from chronic colitis, induced by dextran sulphate sodium (DSS). The course of the disease, intestinal histopathological changes, and in-situ immunological phenotype are scored over 25 days. Our results show that both Q and I3C improved significantly clinical symptoms in moderate DSS colitis, which coincides with a significantly reduced histopathological score. Even in severe DSS colitis I3C, neither Q nor the therapy control 6-thioguanine (6-TG) can prevent a fatal outcome. Moreover, treatment with Q or I3C restored in part DSS-induced loss of epithelial integrity by induction of tight-junction proteins and reduced significantly gut inflammation, as demonstrated by colonoscopy, as well as by immunohistochemistry revealing lower numbers of neutrophils and macrophages. Moreover, the number of Th17 cells is significantly reduced, while the number of Treg cells is significantly increased by treatment with Q or I3C, as well as 6-TG. Q- or I3C-induced amelioration of colitis is not observed in Ahr-/- mice suggesting the requirement of AhR ligation and signalling. Based on the results of this study, plant-derived non-toxic AhR agonists can be considered promising therapeutics in IBD therapy in humans. However, they may differ in terms of efficacy; therefore, it is indispensable to study the dose-response relationship of each individual AhR agonist also with regard to potential adverse effects, since they may also exert AhR-independent effects.