The interaction that binds animals to their microbiota profoundly shapes many aspects of their biology. One such aspect is linear growth at the juvenile stage, especially upon undernutrition. We aim to elucidate the mechanisms that forge the beneficial interaction between the juvenile host and its gut microbiota. Using gnotobiotic flies and mice, we have demonstrated the ability of selected lactobacilli strains to promote linear growth upon chronic undernutrition. These discoveries suggest that the still unknown molecular mechanisms underlying microbiota-mediated linear growth promotion are likely conserved during evolution. We are now probing the mechanistic basis of such phenomena using genetic approaches in Drosophila coupled to functional studies in monoxenic and conventional mice. We believe that this basic knowledge carries lot’s of potential for industrial applications and paves the way to clinical studies to evaluate the potential of lactobacilli-based interventions to support child growth and health upon undernutrition.