Quorum sensing regulates virulence factors in the coral pathogen Vibrio coralliilyticus

The bacterial pathogen Vibrio coralliilyticus causes disease in coral species worldwide. The mechanisms of V. coralliilyticus coral colonization, coral microbiome interactions, and virulence factor production are understudied. In other model Vibrio species, virulence factors like biofilm formation, toxin secretion, and protease production are controlled through a density-dependent communication system called quorum sensing (QS). Comparative genomics indicated that V. coralliilyticus genomes share high sequence identity for most of the QS signaling and regulatory components identified in other Vibrio species. Here, we identify an active QS signaling pathway in two V. coralliilyticus strains with distinct infection etiologies: type strain BAA-450 and coral isolate OCN008. In V. coralliilyticus, the inter-species AI-2 autoinducer signaling pathway in both strains controls expression of the master QS transcription factor and LuxR/HapR homolog VcpR to regulate >300 genes, including protease production, biofilm formation, and two conserved type VI secretion systems (T6SSs). Activation of T6SS1 by QS results in the secretion of effectors and enables interbacterial competition and killing of prey bacteria. We conclude that the QS system in V. coralliilyticus is functional and controls the expression of genes involved in relevant bacterial behaviors typically associated with host infection.IMPORTANCEVibrio coralliilyticus infects many marine organisms, including multiple species of corals, and is a primary causative agent of tissue loss diseases and bacterial-induced bleaching. Here, we investigated a common cell-cell communication mechanism called quorum sensing, which is known to be intimately connected to virulence in other Vibrio species. Our genetic and chemical studies of V. coralliilyticus quorum sensing uncovered an active pathway that directly regulates the following key virulence factors: proteases, biofilms, and secretion systems. These findings connect bacterial signaling in communities to the infection of corals, which may lead to novel treatments and earlier diagnoses of coral diseases in reefs.