The human gastro-intestinal (GI) tract is the primary site of food intake, food perception and food conversion. It represents one of the bodies' metabolically most active organs and is colonized by a myriad of microbes that vastly outnumber the human cells and contribute to nutrient processing. Notably in the colon the most important metabolic conversions of nutrient substrates are realized by the microbial community. Although increasing insight has been obtained in the microbial diversity, there is very limited knowledge of the metabolic function of these microbes, the way the diet affects metabolic fluxes, and how the produced metabolites affect the host. Moreover, it is know that metabolic distortion in the colon leads to significant intestinal health defects (e.g. colon cancer, inflammatory bowel disease (IBD), and irritable bowel syndrome) underpinning the importance of microbial metabolism. While the power of genomics will allow for the analysis of the expression of both microbial and host genes, functional insight is only possible by complementation with a metabolic approach (metabolomics). There is little doubt that nutrition has an impact on the composition and activity of the human gut microbiota and thereby influences human health and well-being. However, the mechanisms behind these processes are largely unknown. Complementary to other WCFS projects focusing on the human intestine, this project aims to optimise large intestinal functionality by addressing the effects of dietary components on the functionality of the microbiota. The project specifically aims to characeterize the relevant microbial metabolites and their fluxes in the colon, link these to the microbial diversity and its spatial distribution, and study the effect on intestinal health. [Objectives and deliverables]: The main objectives of this project entitled "Microbe-mediated gut metabolism" are (i) to analyze the flux and concentration of SCFAs and their impact on the colonic transcriptional response and health parameters, (ii) to determination colonic absorption of microbially produced metabolites, (iii) to characterize the microbial food chains involved in carbohydrate conversion into microbial metabolites; (iv) to develop biomarkers for microbial processes involved in intestinal health; (v) to optimalize and validate in vitro models, (vi) to develop a sampling devices for in situ sampling, and (vii) to determine microbial sulphur metabolism and its modulation by diet. This is realized in three interrelated subprojects addressing a) Metabolite and flux analysis, b) Microbial metabolism and c) Intestinal gut health. Use will be made of stable isotope-labelled substrates, intervention strategies in human volunteers, and investigational tools, including stable isotope probing, spectroscopic analysis and metabolomics, transcriptomics and proteomics, that are linked using pattern recognition. Related to these tools, a sampling device for in situ sampling in the large intestine (a chemical memory chip) will be developed and applied for the determination of microbial metabolites in the colon. Furthermore, sulphur-metabolism will be studied, because of the suspected interaction between carbohydrate and sulphur metabolism. All subprojects are primarily focused on human trials, but will be complemented by in vitro studies to develop models for more throughput screening of food components. [Scientific progress]: The project is a unique combination of the use of stable isotopes and the "~omics" approaches. It combines various relatively unrelated disciplines such as microbial physiology, human physiology and gastroenterology, and incorporates state-of-the-art developments in the biomedical and nanotechnology. The knowledge generated in this project can be exploited to develop foods that have the desirable functionalities in the consumers' intestine, ranging from improvement of nutrient adsorption, to decreased intestinal complaints and perhaps even dietary means of reducing colon cancer and inflammatory disease.