Vibrios are natural inhabitants of aquatic environments and form symbiotic or pathogenic relationships with eukaryotic hosts. Recent studies reveal that the ability of vibrios to form biofilms – i.e. matrix-enclosed, surface-associated communities– depends upon specific structural genes (flagella, pili, and exopolysaccharide biosynthesis) and regulatory processes (two-component regulators, quorum sensing, and c-di-GMP signaling). In this review, we compare and contrast mechanisms and regulation of biofilm formation by Vibrio species, with a focus on Vibrio cholerae, Vibrio parahaemolyticus, Vibrio vulnificus, and Vibrio fischeri. While many aspects are the same, others differ dramatically. Critical questions that remain to be answered regarding the molecular underpinnings of Vibrio biofilm formation will also be discussed.

The bioluminescent bacterium Vibrio fischeri and juveniles of the squid Euprymna scolopes specifically recognize and respond to one another during the formation of a persistent colonization within the host's nascent light-emitting organ. The resulting fully developed light organ contains brightly luminescing bacteria and has undergone a bacterium-induced program of tissue differentiation, one component of which is a swelling of the epithelial cells that line the symbiont-containing crypts. While the luminescence (lux) genes of symbiotic V. fischeri have been shown to be highly induced within the crypts, the role of these genes in the initiation and persistence of the symbiosis has not been rigorously examined. We have constructed and examined three mutants (luxA, luxI, and luxR), defective in either luciferase enzymatic or regulatory proteins. All three are unable to induce normal luminescence levels in the host and, 2 days after initiating the association, had a three-to fourfold defect in the extent of colonization. Surprisingly, these lux mutants also were unable to induce swelling in the crypt epithelial cells. Complementing, in trans, the defect in light emission restored both normal colonization capability and induction of swelling. We hypothesize that a diminished level of oxygen consumption by a luciferase-deficient symbiotic population is responsible for the reduced fitness of lux mutants in the light organ crypts. This study is the first to show that the capacity for bioluminescence is critical for normal cell-cell interactions between a bacterium and its animal host and presents the first examples of V. fischeri genes that affect normal host tissue development.

SummarySuccessful colonization of a eukaryotic host by a microbe involves complex microbe-microbe and microbe-host interactions. Previously, we identified in Vibrio fischeri a putative sensor kinase, RscS, required for initiating symbiotic colonization of its squid host Euprymna scolopes. Here, we analysed the role of rscS by isolating an allele, rscS1, with increased activity. Multicopy rscS1 activated transcription of genes within the recently identified symbiosis polysaccharide (syp) cluster. Wild-type cells carrying rscS1 induced aggregation phenotypes in culture, including the formation of pellicles and wrinkled colonies, in a syp-dependent manner. Colonies formed by rscS1-expressing cells produced a matrix not found in control colonies and largely lost in an rscS1-expressing sypN mutant. Finally, multicopy rscS1 provided a colonization advantage over control cells and substantially enhanced the ability of wildtype cells to aggregate on the surface of the symbiotic organ of E. scolopes; this latter phenotype similarly depended upon an intact syp locus. These results suggest that transcription induced by RscSmediated signal transduction plays a key role in colonization at the aggregation stage by modifying the cell surface and increasing the ability of the cells to adhere to one another and/or to squid-secreted mucus.

SummaryVibrio fischeri is the exclusive symbiont residing in the light organ of the squid Euprymna scolopes . To understand the genetic requirements for this association, we searched a library of V. fischeri transposon insertion mutants for those that failed to colonize E. scolopes . We identified four mutants that exhibited severe defects in initiating colonization. Sequence analysis revealed that the strains contained insertions in four different members of a cluster of 21 genes oriented in the same direction. The predicted gene products are similar to proteins involved in capsule, exopolysaccharide or lipopolysaccharide biosynthesis, including six putative glycosyltransferases. We constructed mutations in five additional genes and found that they also were required for symbiosis. Therefore, we have termed this region syp , for symbiosis polysaccharide. Homologous clusters also exist in Vibrio parahaemolyticus and Vibrio vulnificus , and thus these genes may represent a common mechanism for promoting bacteria-host interactions. Using lacZ reporter fusions, we observed that transcription of the syp genes did not occur under standard laboratory conditions, but could be induced by multicopy expression of sypG , which encodes a response regulator with a predicted s s s s 54 interaction domain. This induction depended on s s s s 54, as a mutation in rpoN abolished syp transcription. Primer extension analysis supported the use of putative s s s s 54 binding sites upstream of sypA , sypI and sypM as promoters. Finally, we found that multicopy expression of sypG resulted in robust biofilm formation. This work thus reveals a novel group of genes that V. fischeri controls through a s s s s 54-dependent response regulator and uses to promote symbiotic colonization.