Indole decreases the virulence of pathogenic vibrios belonging to the Harveyi clade

Indole is a signaling molecule secreted by over 85 species of bacteria, including several Vibrio species, and it has been reported to affect different bacterial phenotypes such as biofilm formation, motility, and virulence. In this study, we aimed at investigating the inter-strain variability of the effect of indole in 12 different strains belonging to the Harveyi clade of vibrios.Indole reduced the virulence of all strains towards gnotobiotic brine shrimp larvae. The survival rate of brine shrimp larvae challenged with vibrios pretreated with indole was increased by 1.3-fold to 1.8-fold. Additionally, indole significantly decreased the biofilm formation in all of the strains, decreased the swimming motility in eight of the strains, and decreased swarming motility in five of the strains. When cultured in the presence of exogenous indole, the mRNA level of the pirA and pirB toxin genes were down-regulated to 65% and 46%, and to 62% and 55% in the AHPND-causing strains Vibrio parahaemolyticus M0904 and Vibrio campbellii S01, respectively.These data indicate that indole has a significant impact on the virulence of different strains belonging to the Harveyi clade of vibrios.Our results suggest that indole signaling is a valid target for the development of novel therapeutics in order to control infections caused by Harveyi clade vibrios in aquaculture.