The microbiota of the human intestinal tract is complex with variable populations of bacteria who are either permanent gut residents (commensal bacteria) or transient inhabitants introduced from the environment. The commensal bacteria are believed to be important for human health due to actions such as protection against pathogens, induction of immune regulatory functions and nutrient processing. Hence, the composition of commensal bacteria is important to preserve colonic health. Ulcerative colitis (UC) is an inflammatory bowel disease and dysbiosis in the composition of commensals has been reported, which could affect colonic health. In the experimental part of this thesis, the fecal microbiota derived from UC patients in either remission or with active disease and healthy subjects was quantified using quantitative Real‐Time PCR (qPCR) to examine the microbiota composition. The results demonstrated that the microbiota composition was different in UC patients in relapse compared to healthy subjects and the difference could be ascribed Gramnegative bacteria, hence indicating that an altered microbiota composition is associated with colonic inflammation. Additionally, results revealed that the microbiota composition in remission either resembled the composition in healthy or in relapse, demonstrating that the microbiota in remission is unstable. The mucus layer lining the epithelium of the intestinal tract is important for the protection of the epithelium in humans. The commensal bacteria that colonize the colonic mucus are suggested to play an important role in stimulating regulatory immune responses compared to luminal bacteria, since they reside closer to the intestinal epithelial cells. The ability of fecal microbiota derived from healthy subjects and UC patients to colonize mucus was examined in a study of this thesis to elucidate, if the adhesion capacity is different depending on disease state. For this purpose, an in vitro dynamic gut model was used. Several bacterial taxa from both lumen and mucus were quantified using qPCR. The results revealed that the bacterial community of the mucus differed from that of the lumen and that lactobacilli and bifidobacteria derived from UC patients had a significant decreased capacity to colonize mucus than observed for similar bacterial groups originating from healthy subjects. This suggests that the inflammatory state in UC may influence the adhesion capacity of commensal bacteria such as beneficial Gram‐positive bacteria lactobacilli and bifidobacteria.