Bacterial filamentation is an in vivo mechanism for cell-to-cell spreading

Intracellular pathogens are challenged with limited space and resources while replicating in a single host cell. Mechanisms for direct invasion of neighboring host cells have been discovered in cell culture, but we lack an understanding of how bacteria directly spread from cell-to-cell in vivo. Here, we describe the discovery of a bacterial species that uses filamentation as an in vivo mechanism for intracellular spreading between the intestinal epithelial cells of its host, the rhabditid nematodeOscheius tipulae. In vitro and in vivo filamentation by this bacterium,Bordetella atropi, requires a highly conserved nutrient-sensing pathway used by divergent bacteria to detect rich conditions and inhibit the divisome. Thus,B. atropiuses a distinct mechanism for cell-to-cell spreading by coopting a pathway that normally regulates bacterial cell size to trigger filamentation inside host cells.