Comparison of Vibrio coralliilyticus virulence in Pacific oyster larvae and corals.

The bacterium Vibrio coralliilyticus has been implicated in mass mortalities of corals and shellfish larvae. However, using corals for manipulative infection experiments can be logistically difficult compared to other model organisms, so we aimed to establish oyster larvae infections as a proxy model. Therefore, this study assessed the virulence of six wild-type V. coralliilyticus strains, and mutants of one strain with deletions of known virulence factors, between Pacific oyster larvae ( Crassostrea gigas ) and Hawaiian rice coral ( Montipora capitata ) infection systems. The wild-type strains tested displayed variable virulence in each system, but virulence levels between hosts were not necessarily comparable. Strains RE98 and OCN008 maintained a medium to high level of virulence across hosts and appeared to be more generalist pathogens. Strain H1, in contrast, was avirulent towards coral but displayed a medium level of virulence towards oyster larvae. Interestingly, the BAA-450 type strain had a medium level of virulence towards coral and was the least virulent to oyster larvae. A comparison of known virulence factors determined that the flagellum, motility or chemotaxis, all of which play a significant role in coral infections, were not crucial for oyster infections with strain OCN008. A genomic comparison of the newly sequenced strain H1 with the other strains tested identified 16 genes potentially specific to coral pathogens that were absent in H1. This is both the first comparison of various V. coralliilyticus strains across infection systems and the first investigation of a strain that is non-virulent to coral. Our results indicate that the virulence of V. coralliilyticus strains in coral is not necessarily indicative of virulence in oyster larvae, and that the set of genes tested are not required for virulence in both model systems. This study increases our understanding of the virulence between V. coralliilyticus strains and helps assess their potential threat to marine environments and shellfish industries.