Abstract
Vibrio anguillarum causes a hemorrhagic septicemia that affects cold- and warm-water adapted fish species. The main goal of this work was to determine the temperature-dependent changes in the virulence factors that could explain the virulence properties of V. anguillarum for fish cultivated at different temperatures. We have found that although the optimal growth temperature is around 25°C, the degree of virulence of V. anguillarum RV22 is higher at 15°C. To explain this result, an RNA-Seq analysis was performed to compare the whole transcriptome profile of V. anguillarum RV22 cultured under low-iron availability at either 25 or 15°C, which would mimic the conditions that V. anguillarum finds during colonization of fish cultivated at warm- or cold-water temperatures. The comparative analysis of transcriptomes at high- and low-iron conditions showed profound metabolic adaptations to grow under low iron. These changes were characterized by a down-regulation of the energetic metabolism and the induction of virulence-related factors like biosynthesis of LPS, production of hemolysins and lysozyme, membrane transport, heme uptake, or production of siderophores. However, the expression pattern of virulence factors under iron limitation showed interesting differences at warm and cold temperatures. Chemotaxis, motility, as well as the T6SS1 genes are expressed at higher levels at 25°C than at 15°C. By contrast, hemolysin RTX pore-forming toxin, T6SS2, and the genes associated with exopolysaccharides synthesis were preferentially expressed at 15°C. Notably, at this temperature, the siderophore piscibactin system was strongly up-regulated. In contrast, at 25°C, piscibactin genes were down-regulated and the vanchrobactin siderophore system seems to supply all the necessary iron to the cell. The results showed that V. anguillarum adjusts the expression of virulence factors responding to two environmental signals, iron levels and temperature. Thus, the relative relevance of each virulence factor for each fish species could vary depending on the water temperature. The results give clues about the physiological adaptations that allow V. anguillarum to cause infections in different fishes and could be relevant for vaccine development against fish vibriosis.
Highlights
Vibrio anguillarum is a marine bacterium inhabitant of the estuarine and marine coastal ecosystems worldwide
Thereby, in order to assay the virulence properties of V. anguillarum at two different water temperatures (15 and 25◦C), S. senegalensis was chosen as a fish model, since it is a species that is naturally exposed to water temperatures from 12 to 26◦C (Arjona et al, 2010)
The results show that the fish mortality observed after infection with V. anguillarum RV22 significantly increase (p = 0.006) at cold-water temperature (Figure 1B)
Summary
Vibrio anguillarum is a marine bacterium inhabitant of the estuarine and marine coastal ecosystems worldwide It is an important fish pathogen since it is the etiological agent of classical vibriosis in warm- and cold-water fish species leading to high mortalities and economic losses in aquaculture (Toranzo et al, 2017). V. anguillarum survives in a cultivable form for more than a year in saline water (Hoff, 1989) where darkness, coldness (around 5◦C), and anaerobiosis are environmental factors that help the pathogen to survive (Eguchi et al, 2000) This favors the dissemination and presence of V. anguillarum in seawater and sediments in all seasons (Muroga et al, 1986; Maeda et al, 2003)
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