The gut microbiota plays a crucial role in the maturation of the intestinal mucosal immune system of its host(1,2). Within the thousand bacterial species present in the intestine, the symbiont segmented filamentous bacterium(SFB) is unique in its ability to potently stimulate the post-natal maturation of the B-and T-cell compartments and induce a striking increase in the small-intestinal Th17 responses(3-5). Unlike other commensals, SFB intimately attaches to absorptive epithelial cells in the ileum and cells overlying Peyer's patches(6,7). This colonization does not result in pathology; rather, it protects the host from pathogens(4). Yet, little is known about the SFB-host interaction that underlies the important immunostimulatory properties of SFB, because SFB have resisted in vitro culturing for more than 50 years. Here we grow mouse SFB outside their host in an SFB-host cell co-culturing system. Single-celled SFB isolated from mono-colonized mice undergo filamentation, segmentation, and differentiation to release viable infectious particles, the intracellular offspring, which can colonize mice to induce signature immune responses. In vitro, intracellular offspring can attach to mouse and human host cells and recruit actin. In addition, SFB can potently stimulate the upregulation of host innate defence genes, inflammatory cytokines, and chemokines. In vitro culturing thereby mimics the in vivo niche, provides new insights into SFB growth requirements and their immunostimulatory potential, and makes possible the investigation of the complex developmental stages of SFB and the detailed dissection of the unique SFB-host interaction at the cellular and molecular levels.