Chapter Number 1 The ECSIM Concept (Environmental Control 2 System for Intestinal Microbiota) and Its 3 Derivative Versions to Help Better 4 Understand Human Gut Biology 5 Jean-François Brugère 1 , David Féria-Gervasio 1 , 6 Zsolt Popse 2 , William Tottey 1 and Monique Alric 1 7 1 Université d’Auvergne, Clermont-Université, Clermont-Ferrand, 8 2 Global Process Concept, La Rochelle, 9 France 10 1. Introduction 11 Each meal reminds us of our energy needs. As heterotrophs, we have a whole set of organs 12 for food digestion and conversion into substances usable by our body. The breakdown of 13 food (mainly proteins, carbohydrates and lipids) takes place in the gastrointestinal tract 14 (GIT) which assimilates these elements and transfers them into the bloodstream. In a 15 simplified manner, these digestion and absorption functions are provided by the following 16 organs: the mouth, oesophagus, stomach, the small intestine (including the duodenum, 17 jejunum and ileum), the large intestine or colon (formed from the right, transverse and left 18 colon), ending with the rectum and anus. Various functions are required to facilitate the 19 hydrolysis of food. They include a combination of physical factors (temperature, pH, 20 grinding and friction, ...), chemical factors (acid secretions, bile salts, ...) and enzymes 21 (salivary, gastric or pancreatic enzymes,…). Colonic physiology also benefits in parallel 22 from a real contribution by micro-organisms, specifically bacteria. 23 Several factors contribute to our interest in the human digestive tract. For example, in 24 Europe colorectal cancer is the second cause of cancer in women and the third in men 25 (Boyle & Ferlay, 2005). Some pathologies can be chronic, infectious or even mortal. The 26 digestive tract is also a simple and practical way of giving chemotherapy treatment, whether 27 the pathology is of digestive origin or not. 28 Proposing more or less advanced simulation systems of the GIT, therefore, can overcome 29 certain ethical, technical and/or financial difficulties in research. Furthermore, it is 30 interesting to model the way the digestive tract works, in order to test various elements 31 independently and/or concomitantly: such as drugs, food/nutrients, microbial agents, even 32 physiological and physical elements. 33 Animal models work particularly well if they have a similar anatomical/physiological GIT. 34 Rodent models, however, are used more in the laboratory for practical reasons. In order to 35 enhance the resemblance to humans, "humanized" animal GITs have be developed from 36 germ-free, newborn animals turned into gnotobiotic animals after being seeded by a human 37