A key role of the gut microbiota in the establishment and maintenance of health, as well as in the pathogenesis of disease, has been identified over the past two decades. One of the primary modes by which the gut microbiota interacts with the host is by means of metabolites, which are small molecules that are produced as intermediate or end products of microbial metabolism. These metabolites can derive from bacterial metabolism of dietary substrates, modification of host molecules, such as bile acids, or directly from bacteria. Signals from microbial metabolites influence immune maturation, immune homeostasis, host energy metabolism and maintenance of mucosal integrity. Alterations in the composition and function of the microbiota have been described in many studies on IBD. Alterations have also been described in the metabolite profiles of patients with IBD. Furthermore, specific classes of metabolites, notably bile acids, short-chain fatty acids and tryptophan metabolites, have been implicated in the pathogenesis of IBD. This Review aims to define the key classes of microbial-derived metabolites that are altered in IBD, describe the pathophysiological basis of these associations and identify future targets for precision therapeutic modulation.

ObjectivesThe relevance of spatial composition in the microbial changes associated with UC is unclear. We coupled luminal brush samples, mucosal biopsies and laser capture microdissection with deep sequencing of the gut microbiota to develop an integrated spatial assessment of the microbial community in controls and UC.DesignA total of 98 samples were sequenced to a mean depth of 31 642 reads from nine individuals, four control volunteers undergoing routine colonoscopy and five patients undergoing surgical colectomy for medically-refractory UC. Samples were retrieved at four colorectal locations, incorporating the luminal microbiota, mucus gel layer and whole mucosal biopsies.ResultsInterpersonal variability accounted for approximately half of the total variance. Surprisingly, within individuals, asymmetric Eigenvector map analysis demonstrated differentiation between the luminal and mucus gel microbiota, in both controls and UC, with no differentiation between colorectal regions. At a taxonomic level, differentiation was evident between both cohorts, as well as between the luminal and mucosal compartments, with a small group of taxa uniquely discriminating the luminal and mucosal microbiota in colitis. There was no correlation between regional inflammation and a breakdown in this spatial differentiation or bacterial diversity.ConclusionsOur study demonstrates a conserved spatial structure to the colonic microbiota, differentiating the luminal and mucosal communities, within the context of marked interpersonal variability. While elements of this structure overlap between UC and control volunteers, there are differences between the two groups, both in terms of the overall taxonomic composition and how spatial structure is ascribable to distinct taxa.