BackgroundWhite Syndrome (WS), a general term for scleractinian coral diseases with acute signs of advancing tissue lesions often resulting in total colony mortality, has been reported from numerous locations throughout the Indo-Pacific, constituting a growing threat to coral reef ecosystems.Methodology/Principal FindingsBacterial isolates were obtained from corals displaying disease signs at three WS outbreak sites: Nikko Bay in the Republic of Palau, Nelly Bay in the central Great Barrier Reef (GBR) and Majuro Atoll in the Republic of the Marshall Islands, and used in laboratory-based infection trials to satisfy Henle-Koch's postulates, Evan's rules and Hill's criteria for establishing causality. Infected colonies produced similar signs to those observed in the field following exposure to bacterial concentrations of 1×106 cells ml−1. Phylogenetic 16S rRNA gene analysis demonstrated that all six pathogens identified in this study were members of the γ-Proteobacteria family Vibrionacae, each with greater than 98% sequence identity with the previously characterized coral bleaching pathogen Vibrio coralliilyticus. Screening for proteolytic activity of more than 150 coral derived bacterial isolates by a biochemical assay and specific primers for a Vibrio family zinc-metalloprotease demonstrated a significant association between the presence of isolates capable of proteolytic activity and observed disease signs.Conclusion/SignificanceThis is the first study to provide evidence for the involvement of a unique taxonomic group of bacterial pathogens in the aetiology of Indo-Pacific coral diseases affecting multiple coral species at multiple locations. Results from this study strongly suggest the need for further investigation of bacterial proteolytic enzymes as possible virulence factors involved in Vibrio associated acute coral infections.

Vibrio shiloi, the causative agent of bleaching of the coral Oculina patagonica in the Mediterranean Sea, is present in all bleached O. patagonica corals in the summer (25-30 degrees C), but can be not detected in the coral during the winter (16-20 degrees C). Furthermore, the pathogen can not survive in O. patagonica at temperatures below 20 degrees C. Using fluorescence in situ hybridization (FISH) with a V. shiloi-specific oligonucleotide probe, we found that the marine fireworm Hermodice caranculata is a winter reservoir for V. shiloi. Worms taken directly from the sea during the winter contained approximately 10(8) V. shiloi per worm by FISH analysis. However, colony-forming units (cfu) revealed only 4.1-18.3 x 10(4) V. shiloi per worm, indicating that approximately 99.9% of them were in the viable-but-not-culturable (VBNC) state. When worms were infected with V. shiloi, most of the bacteria adhered to the worm within 24 h and then penetrated into epidermal cells. By 48 h, less than 10(-4) of the intact V. shiloi in the worm gave rise to colonies, suggesting that they differentiated inside the worm into the VBNC state. When worms infected with V. shiloi were placed in aquaria containing O. patagonica, all of the corals showed small patches of bleached tissue in 7-10 days and total bleaching in 17 days. This is the first report of a reservoir and vector for a coral disease.