Microscopic colitis (MC) is an inflammatory bowel disease characterized by chronic non-bloody watery diarrhoea and a macroscopically normal colonic mucosa upon endoscopic exploration (colonoscopy). The diagnosis is performed by microscopic examination of mucosal biopsies that reveal specific histopathological change. Between 4-20% of patients with chronic non-bloody diarrhoea who undergo colonoscopy with serial biopsies are diagnosed with MC. It has long been hypothesized that the microbiome plays a key role in the pathogenesis of MC. In patients with collagenous colitis, faecal stream diversion results in inflammation and histological remission, followed by disease relapse after intestinal transit is reconstructed. Moreover, studies carried out with faecal samples obtained after colonoscopy have demonstrated microbiome changes (reduced alpha diversity and higher microbial dysbiosis index) in patients with active MC. To avoid potential bias due to the effect of colonic lavage prior to colonoscopy in microbiota composition, the researchers of the present study previously evaluated the microbiome in faecal samples obtained before the diagnostic colonoscopy in patients with active MC. The results confirmed a reduced alpha diversity in diarrhoea groups; however, there were no differences between MC, bile-acid diarrhoea and functional diarrhoea. The microbial dysbiosis index was significantly higher in MC compared to the other diarrheal groups, but no bacterial species showed a significantly different relative abundance. On the other hand, the risk of colorectal cancer (CRC) or adenoma seems to be reduced in MC compared to controls. Growing evidence suggests microbial dysbiosis is a crucial environmental factor in the initiation of precancerous lesions of CRC such as adenomas. The objective of the current multicentric prospective study is to assess the differences in the mucosa adherent intestinal microbiome between patients with MC, non-MC chronic diarrhoea, healthy controls and patients with advanced colon adenomas. In addition to the study of the microbiome, sociodemographic variables, history of drug usage, diets and specific characteristics of diarrhoea will be collected. The hypothesis of the present study is that CM presents a specific mucosa adherent intestinal bacterial profile that may be relevant in the pathogenesis of the disease and that, additionally, may also play a protective role against the development of CRC and adenomas.