The expression of virulence factors in vibrio anguillarum is dually regulated by iron levels and temperature

Vibrio anguillarumcauses a hemorrhagic septicemia that affects cold- and warm-wateradapted fish species. The main goal of this work was to determine the temperature-dependent changes in the virulence factors that could explain the virulence propertiesofV. anguillarumfor fish cultivated at different temperatures. We have found thatalthough the optimal growth temperature is around 25◦C, the degree of virulence ofV. anguillarumRV22 is higher at 15◦C. To explain this result, an RNA-Seq analysiswas performed to compare the whole transcriptome profile ofV. anguillarumRV22cultured under low-iron availability at either 25 or 15◦C, which would mimic theconditions thatV. anguillarumfinds during colonization of fish cultivated at warm- orcold-water temperatures. The comparative analysis of transcriptomes at high- andlow-iron conditions showed profound metabolic adaptations to grow under low iron.These changes were characterized by a down-regulation of the energetic metabolismand the induction of virulence-related factors like biosynthesis of LPS, productionof hemolysins and lysozyme, membrane transport, heme uptake, or production ofsiderophores. However, the expression pattern of virulence factors under iron limitationshowed 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 associatedwith 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 vanchrobactinsiderophore system seems to supply all the necessary iron to the cell. The resultsshowed thatV. anguillarumadjusts the expression of virulence factors responding to twoenvironmental signals, iron levels and temperature. Thus, the relative relevance of eachvirulence factor for each fish species could vary depending on the water temperature.The results give clues about the physiological adaptations that allowV. anguillarumtocause infections in different fishes and could be relevant for vaccine development againstfish vibriosisThis work was supported by grants AGL2015-63740-C2-1-R and RTI2018-093634-B-C21 (AEI/FEDER, EU) from the State Agency for Research (AEI) of Spain and cofunded by the FEDER Programme from the European Union. The support of Xunta de Galicia (Spain) with grant GRC2018/018 is also acknowledgedS