Abstract
Bacterial flagellar motility is a complex cellular behavior required for the colonization of the light-emitting organ of the Hawaiian bobtail squid, Euprymna scolopes, by the beneficial bioluminescent symbiont Vibrio fischeri. We characterized the basis of this behavior by performing (i) a forward genetic screen to identify mutants defective in soft-agar motility, as well as (ii) a transcriptional analysis to determine the genes that are expressed downstream of the flagellar master regulator FlrA. Mutants with severe defects in soft-agar motility were identified due to insertions in genes with putative roles in flagellar motility and in genes that were unexpected, including those predicted to encode hypothetical proteins and cell division–related proteins. Analysis of mutants for their ability to enter into a productive symbiosis indicated that flagellar motility mutants are deficient, while chemotaxis mutants are able to colonize a subset of juvenile squid to light-producing levels. Thirty-three genes required for normal motility in soft agar were also downregulated in the absence of FlrA, suggesting they belong to the flagellar regulon of V. fischeri. Mutagenesis of putative paralogs of the flagellar motility genes motA motB, and fliL revealed that motA1 motB1, and both fliL1 and fliL2, but not motA2 and motB2, likely contribute to soft-agar motility. Using these complementary approaches, we have characterized the genetic basis of flagellar motility in V. fischeri and furthered our understanding of the roles of flagellar motility and chemotaxis in colonization of the juvenile squid, including identifying 11 novel mutants unable to enter into a productive light-organ symbiosis.
Highlights
Flagellar motility is an environmentally regulated behavior by which a bacterium propels itself through its surroundings, directed by behavior-modifying machinery such as the chemotaxis system (Adler 1966; Henrichsen 1972; reviewed in Macnab 1996; and McCarter 2006)
MicrobiologyOpen published by John Wiley & Sons Ltd
While the process of flagellar motility is difficult to study in most host–microbe interactions, the symbiosis between the bioluminescent, gram-negative bacterium Vibrio fischeri and its host the Hawaiian bobtail squid, Euprymna scolopes, is an ideal model in which to study how this critical behavior mediates symbiotic initiation
Summary
Flagellar motility is an environmentally regulated behavior by which a bacterium propels itself through its surroundings, directed by behavior-modifying machinery such as the chemotaxis system (Adler 1966; Henrichsen 1972; reviewed in Macnab 1996; and McCarter 2006). The use of multiple polar flagella in V. fischeri is unique even among the well-studied Vibrio species (McCarter 2006): Vibrio cholerae bears a single polar flagellum and V. parahaemolyticus presents either a single sheathed polar flagellum or multiple unsheathed lateral flagella, depending on its environment (Shinoda and Okamoto 1977; Freter et al 1981; McCarter and Silverman 1990) These structural differences suggest that flagellar biosynthesis is uniquely adapted in this genus. Several genetic studies have identified the genes involved in proper flagellar elaboration in bacteria with polar flagellar systems (Kim and McCarter 2000; Overhage et al 2007) In these bacteria, additional proteins are important for motility, including the regulators FlhF and FlhG, which control flagellar number in V. cholerae (Correa et al 2005).
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