Indole signaling is viewed as a potential target for antivirulence therapy against antibiotic-resistant pathogens because of its link with the production of virulence factors. This study examined the antimicrobial and antivirulence properties of 44 indoles toward Vibrio campbellii. Based on the results, 17 halogenated indole analogues were selected, as they significantly improved the survival of brine shrimp larvae challenged with V. campbellii. Specifically, 6-bromoindole, 7-bromoindole, 4-fluoroindole, 5-iodoindole, and 7-iodoindole showed a high protective effect, improving the survival of brine shrimp to over 80% even at a low concentration of 10 μM. To explore the impact of selected indole analogues on bacterial virulence phenotypes, swimming motility, biofilm formation, protease activity, and hemolytic activity of V. campbellii were determined. The results showed that all of the 17 selected indole analogues decreased swimming motility at both 10 μM and 100 μM. Most of the indole analogues decreased biofilm formation at a concentration of 100 μM. In contrast, only a slightly decreased protease activity and no effect on hemolytic activity were observed at both concentrations. To our knowledge, this is the first study of the structure-activity relation of halogenated indole analogues with respect to virulence inhibition of a pathogenic bacterium in an in vivo host model system, and the results demonstrate the potential of these compounds in applications aiming at the protection of shrimp from vibriosis, a major disease in aquaculture. IMPORTANCE Bacterial diseases are a major problem in the aquaculture industry. In order to counter this problem, farmers have been using antibiotics, and this has led to the evolution and spread of antibiotic resistance. In order for the aquaculture industry to further grow in a sustainable way, novel and sustainable methods to control diseases are needed. We previously reported that indole signaling is a valid target for the development of novel therapies to control disease caused by Vibrio campbellii and related bacteria, which are among the major bacterial pathogens in aquaculture. In the present study, we identified indole analogues that are more potent in protecting brine shrimp (a model organism for shrimp) from V. campbellii. To our knowledge, this is the first study of the structure-activity relation of halogenated indole analogues with respect to virulence inhibition of a pathogenic bacterium in an in vivo host model system.
Keywords: antivirulence therapy; indole signaling; quorum sensing; shrimp; virulence.