The antivirulence effect of 5-iodoindole on Vibrio splendidus AJ01 towards Apostichopus japonicus

<i>Vibrio splendidus</i> AJ01 is the primary pathogen of <i>Apostichopus japonicus</i>, resulting in huge economic losses. In the present study, we found that 5-iodoindole exhibits a remarkable protective effect on <i>A. japonicus</i>, the survival rate of which can be improved to 93.75%. To explore the impact of 5-iodoindole on <i>V. splendidus</i> AJ01, bacterial growth and virulence factors were determined. Results showed that 5-iodoindole significantly decreased swimming motility, hemolytic activity and protease activity but had minimal effect on the overall growth of the bacterium. Furthermore, a differentially expressed protein was downregulated and identified as elongation factor thermo unstable protein (EF-Tu) via SDS-PAGE combined with LC-MS/MS. Moreover, 5-iodoindole had impact on a large number of genes at RNA level, such as genes related to two-component system, ABC transporters, flagella, and chemotaxis. In addition, 20 µM 5-iodoindole could upregulated the mRNA level of immune related genes in <i>A. japonicus</i>, including IL-17, VEGF and MMP1. Our results indicated 5-iodoindole protect <i>A. japonicus</i> through a two-pronged approach: not only inhibiting bacterial virulence but also enhancing the host's immunity. This not only addresses the limitations of traditional indole but also provides a green alternative to antibiotics for protecting <i>A. japonicus</i> from infections caused by <i>V. splendidus</i> AJ01.