Comparative genomics reveals modular virulence repertoires and extensive horizontal gene transfer in Vibrio species associated with white syndrome of Porites cylindrica.

Coral white syndrome (WS) is a widespread condition characterized by tissue loss and skeletal exposure, with multiple bacterial pathogens - particularly Vibrio species - implicated in its aetiology. To investigate the genomic basis of potential virulence in Vibrio associated with WS of Porites cylindrica, we isolated 57 Vibrio strains from healthy and diseased coral tissues collected from reefs in Guam. Thirteen representative strains from five dominant species - Vibrio coralliilyticus, Vibrio owensii, Vibrio tubiashii, Vibrio harveyi and Vibrio tetraodonis - were selected for whole-genome sequencing using Oxford Nanopore Technology. Comparative genomic analyses revealed a conserved repertoire of extracellular enzymes, including haemolysins, cytolysins, metalloproteases and subtilisin-like peptidases, alongside lineage-specific toxin and regulatory modules. Variation in secretion systems (T1SS-T6SS), particularly in T3SS, T4SS and T6SS subtypes, reflected diversification in host interactions and competitive capacities across species. Mobile genetic elements, including plasmids and prophages, contributed additional virulence-associated genes and secretion clusters, underscoring the role of horizontal gene transfer in shaping the accessory genome content. Notably, V. coralliilyticus harboured homologues of the cholera toxin-associated genes ace and zot, although their relatively low aa similarity to characterized Vibrio cholerae toxins suggests that their functional roles remain uncertain. Together, these comparative genomic analyses suggest that virulence-associated gene repertoires are broadly distributed among coral-associated Vibrio and are organized as a conserved ecological core complemented by flexible, horizontally acquired accessory modules. These findings provide hypotheses regarding the genomic basis of opportunistic pathogenicity that will require future functional validation. This study provides the first comparative genomic framework for Vibrio associated with P. cylindrica, providing a foundation for future investigations into how genomic plasticity and modular virulence gene repertoires may contribute to opportunistic disease dynamics in coral reef ecosystems.