The impact of Vibrio fischeri strain variation on host colonization

Abstract

Strain-level epidemiology is a key approach to understanding the mechanisms underlying establishment of any host-microbe association. The squid-vibrio light organ symbiosis has proven to be an informative and tractable experimental model in which to discover these mechanisms because it involves only one bacterial species, Vibrio fischeri. In this horizontally transmitted symbiosis, the squid presents nutrients to the bacteria located in a bilobed light-emitting organ, while the symbionts provide bioluminescence to their host. To initiate this association, V. fischeri cells go through several distinct stages: from free-living in the bacterioplankton, to forming a multicellular aggregation near pores on the light organ's surface, to migrating through the pores and into crypts deep in the light organ, where the symbiont population grows and luminesces. Because individual cells must successfully navigate these distinct regions, phenotypic differences between strains will have a strong impact on the composition of the population finally colonizing the squid. Here we review recent advances in our understanding of behavioral characteristics that differentially drive a strain's success, including its effectiveness of aggregation, the rapidity with which it reaches the deep crypts, and its deployment of type VI secretion.

Fig. 1.

(Upper) Different V. fischeri strains, present in the environment, (I.) form aggregates around the three pores (p) of the light organ, then detach and (II.) migrate through the duct (d), and finally (III.) arrive and colonize deep in the crypts (c) of the light organ. Green and red ovals represent different V. fischeri strains in the environment, and the dashed arrows are their trajectories. (Lower) Confocal images corresponding to the different stages of bacterial behavior along the colonization path. Bacteria are GFP or RFP labeled; nuclei of host tissue (blue) were stained with TOTO-3. Cells of V. fischeri are aggregated (in I.), migrating through the ducts (in II.), and colonizing the light organ crypts (in III.); bar = 50 mm. Note that each crypt is typically colonized by a single bacterium (green or red) that initiates the symbiont population therein.