The beneficial microbes that live in and on us play many important roles in our health. In mammals, these microbial communities are assembled after birth, particularly before weaning. Intestinal microbes do more than just help digest food or produce metabolites. They also contribute to the development of organ structures and the immune system. To better understand this process, researchers used mice with a specifically constructed, simplified gut microbiome. Mice without a microbiome, called germ-free, have significantly impaired immunity, but giving these adult mice a simple microbiome of 14 strains was enough for them to establish an immune response. The addition of microbes stimulated maturation of the intestinal blood vessel system and activated both innate and adaptive immunity. Specifically, germ-free mice could not move critical immune cells, neutrophils, into the gut when infected with Citrobacter rodentium, but even the simplified microbiome allowed the vascular system to expand enough for neutrophils to move in and attack that pathogen. While the human microbiome is much more complex than the one employed in this study, these results suggest that microbial communities can promote maturation of the intestinal vascular and immune system even in adult animals.