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

Aim 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. Methods and Results 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. Conclusions These data indicate that indole has a significant impact on the virulence of different strains belonging to the Harveyi clade of vibrios. Significance and Impact of the Study 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.

Automated summary

The study investigated the effect of indole on 12 different strains of vibrios in the Harveyi clade, finding that indole reduced virulence, biofilm formation, and motility in these strains. Additionally, indole down-regulated toxin genes in AHPND-causing strains, highlighting its potential as a target for novel therapeutics in controlling infections in aquaculture.