Abstract
Quorum sensing, bacterial cell-to-cell communication with small signal molecules, controls the virulence of many pathogens. In contrast to other vibrios, neither the VanI/VanR acylhomoserine lactone quorum sensing system, nor the three-channel quorum sensing system affects virulence of the economically important aquatic pathogen Vibrio anguillarum. Indole is another molecule that recently gained attention as a putative signal molecule. The data presented in this study indicate that indole signaling and the alternative sigma factor RpoS have a significant impact on the virulence of V. anguillarum. Deletion of rpoS resulted in increased expression of the indole biosynthesis gene tnaA and in increased production of indole. Both rpoS deletion and the addition of exogenous indole (50–100 µM) resulted in decreased biofilm formation, exopolysaccharide production (a phenotype that is required for pathogenicity) and expression of the exopolysaccharide synthesis gene wbfD. Further, indole inhibitors increased the virulence of the rpoS deletion mutant, suggesting that indole acts downstream of RpoS. Finally, in addition to the phenotypes found to be affected by indole, the rpoS deletion mutant also showed increased motility and decreased sensitivity to oxidative stress.
Highlights
Vibrio anguillarum is the causative agent of vibriosis, a fatal haemorrhagic septicaemia affecting many aquatic organisms [1]
Impact of RpoS on indole production in V. anguillarum RpoS had previously been reported to increase indole production in E. coli by inducing the expression of the tryptophanase gene tnaA [18] and we investigated the impact of rpoS deletion on indole production in V. anguillarum
In contrast to what has been reported for E. coli, indole production was significantly increased in the rpoS mutant when compared to wild-type V. anguillarum (Figure 1A)
Summary
Vibrio anguillarum is the causative agent of vibriosis, a fatal haemorrhagic septicaemia affecting many aquatic organisms (fish, crustaceans as well as mollusks) [1]. The bacterium is a major pathogen of aquaculture organisms, causing significant economic losses in the aquaculture industry [2]. Several (putative) virulence factors have been identified, for many of these factors, the specific role in disease is not yet known. Three factors that have been reported to be essential for pathogenicity include the iron uptake system involving the siderophore anguibactin [3,4], chemotactic motility (which is required for entry into the host) [5,6] and exopolysaccharide production (which is required for attachment to the host) [7]. The bacterium produces a number of other (putative) virulence factors, including haemolysin, lipase and protease [8,9,10]. Whether or not these factors are really essential for pathogenicity is currently not clear
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