Final Report Summary - MUTFLYGUTBACT (Host-intestinal bacteria mutualism: "Learning on the fly") Our body is colonized by a vast number of microbes, collectively referred to as the human microbiota. Microbiotas are found in virtually any living organism, and it is now well established that they directly influence many aspects of our physiology. In return, our microbiotas derive benefit from the association with us by inhabiting nutrient rich environments such as our intestine. When deregulated this mutually beneficial relationship result in pathologies. Nutrition is one factor by which the host-microbiota equilibrium can be altered. For example, children suffering from chronic and severe malnutrition in poor and developing countries show signs of immature intestinal microbiota, which is associated with severe growth defects and in extreme cases, childhood mortality. Until recently no study had informed us to what extend and how the intestinal microbiota govern juvenile growth. So we decided to tackle this question by using animal model organisms. We have recently revealed that the intestinal microbiota directly influence animal juvenile growth by supporting the production and activity of growth factors. In addition, we identified specific bacterial strains present in the intestinal tract of animals with potent abilities to impact their juvenile growth and growth factors production and activity. Based on these discoveries our current research projects now aims at deciphering the mechanisms by which such bacterial strains influence juvenile growth and its underlying hormonal regulation. This basic research work is particularly relevant for health in general and disease related to undernutrition in particular, and we think that the fundamental knowledge generated by our research program will be a decisive asset in the establishment of potential health claims for use of bacterial strains to support child growth and health when facing undernutrition.