| Vibrio fischeri is a Gram-negative, bioluminescent marine bacterium found both free-living and associated with the light organs of several species of fishes and squids. The symbiosis between V. fischeri and the Hawaiian bobtail squid, Euprymna scolopes, begins anew with each newly hatched juvenile squid. Symbionts from the surrounding seawater form mucus-bound aggregates on the light-organ surface, and then migrate to deep epithelium-lined crypts. Flagellar motility is essential for this symbiotic initiation, while chemotaxis mediates efficient colonization by V. fischeri. However, these important behaviors are poorly defined in V. fischeri. Using both forward and reverse genetic approaches, we first dissected the genetic basis of these behaviors, including the identification of novel motility-associated genes and the investigation of putative flagellar-gene paralogs. Using complementary genetic and imaging techniques, we then characterized the physical and chemical cues to which V. fischeri responds during migration into the light-organ crypts to understand the roles of both in symbiotic initiation. Models of natural initiation are under-represented in studies of host-microbe interactions, so the complexity observed during initiation of the squid-vibrio symbiosis informs a greater understanding of the mechanisms by which microbes colonize a host.;To better grasp how V. fischeri senses and responds to the environment presented by the host, we sought to clarify the complex chemotactic repertoire of V. fischeri. We identified the ligands for two chemoreceptors: VfcA, which mediates chemotaxis towards amino acids; and VfcB, which senses several sugars, including chitobiose, a chitin-breakdown product found in the host environment. This study also defines a novel approach to determine the ligands sensed by chemoreceptors that can be applied across bacterial species.;The flagella of V. fischeri, as well as several important human pathogens, are encased in a membranous sheath of unknown function. Our work identified a novel role for the sheathed flagellum in the release of lipopolysaccharide, an immunogenic molecule, into the surrounding environment upon flagellar rotation. We demonstrated that this shed lipopolysaccharide was required for induction of host epithelial apoptosis. This phenomenon also informs the study of pathogenic bacteria with sheathed flagella, and opens new avenues of research into the biophysics of sheathed flagellar function. |
