Abstract

Vibrio bivalvicida VPAP30 is a highly virulent pathogen of the scallop Argopecten purpuratus, causing severe larval mortalities in hatcheries. Indole, a tryptophan-derived bacterial signaling molecule, has been proposed as a virulence-attenuating agent in vibrios. To propose sustainable alternatives to antibiotics, we evaluated the effect of exogenous indole (100, 200 and 500 mu M) on multiple virulence traits of V. bivalvicida VPAP30, including gene expression, motility, extracellular product (ECP) secretion, proteolytic activity, cytotoxicity toward scallop hemocytes, and pathogenicity in vivo. Transcriptional analyses of virulence-related genes at 4, 8 and 24 h revealed a concentration- and time-dependent modulation, with 500 mu M indole consistently repressing flagellar and motility-associated genes and suppressing protease secretion after 24 h of exposure. Consistently, 500 mu M indole impaired swimming motility, reduced ECP protein content by >90 %, and abolished detectable protease activity. Cytotoxicity assays showed that V. bivalvicida VPAP30 strain pre-exposed to 500 mu M indole induced similar to 50 % less hemocyte lysis compared with untreated controls, and lower indole concentration. Importantly, in vivo infection assays demonstrated that pre-exposure of V. bivalvicida VPAP30 to 500 mu M indole significantly improved survival of both scallop juveniles and larvae, reducing cumulative mortalities to levels comparable with controls. Together, these findings demonstrate that indole attenuates the virulence of V. bivalvicida VPAP30 through concentration-dependent suppression of motility and protease secretion, highlighting its potential as a non-antibiotic strategy to control vibriosis and enhance hatchery sustainability in scallop aquaculture.