Understanding the mechanisms of intracellular filamentation by bacteria

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PROJECT SUMMARY Bacteria often change their cell shape as a fitness strategy to survive or thrive in diverse environments. Filamentation is one such morphological strategy, whereby a bacterium dramatically increases in cell size as it undergoes longitudinal growth without septation. Many bacteria have been observed to undergo filamentation, but the purpose of these filaments in a host animal is often unknown and the genetic mechanisms governing this morphological change are largely understudied, outside of the canonical SOS response. This proposal utilizes an innovative model system to study bacterial filamentation in the context of a host animal, which consists of a facultative intracellular bacterium, Bordetella atropi, that filaments in the genetic model organisms Oscheius tipulae and Caenorhabditis elegans. Strikingly, B. atropi uses filamentation as an adaptive response to nutrients for cell-to-cell spreading: after invasion of an intestinal epithelial cell, B. atropi converts from a coccobacillus to a filamentous morphology and these filaments invade multiple neighboring epithelial cells. This filamentation by B. atropi was found to be independent of the canonical SOS response but required a conserved nutrient-sensing pathway, the glucolipid pathway, which produces and utilizes the bacterial metabolite UDP-glucose. Divergent bacteria use the glucolipid pathway to detect rich culture conditions and delay the divisome resulting in moderately larger progeny after binary division. Taken together, these discoveries suggest that B. atropi detects the rich intracellular environment of its host cell to trigger filamentation, allowing it to invade neighboring cells. This proposal will further elucidate the mechanism that the glucolipid pathway uses to inhibit the bacterial divisome and initiate filamentation, uncover the role that host metabolites play in triggering the phenotype, and discover other genes/pathways controlling intracellular filamentation by bacteria. First, the genetic mechanism that glucolipid pathway members use to constitutively inhibit septation will be investigated in B. atropi and this pathway will be tested in related bacterial species for a role in controlling filamentation. Second, given that the glucolipid pathway can delay binary division in bacteria grown on rich media, B. atropi will be tested for dependency on host glucose levels and a rich intracellular environment to induce filamentation. Finally, a transposon mutagenesis screen will identify a majority of bacterial genes required for intracellular filamentation in O. tipulae. The results established here will serve as a technical roadmap for determining whether other bacteria can use the glucolipid pathway or other novel genetic pathways to induce filamentation. Given the importance of filamentation to bacterial function and survival in a variety of niches, including in the context of a host animal, this work will increase our understanding of both the environmental triggers and genetic mechanisms that lead this vital morphological change.