Selection of Vibrio crassostreae relies on a plasmid expressing a type 6 secretion system cytotoxic for host immune cells

• Published in: Environmental Microbiology
• Main Authors: Piel, Damien, Bruto, Maxime, James, Adèle, Labreuche, Yannick, Lambert, Christophe, Janicot, Adrian, Chenivesse, Sabine, Petton, Bruno, Wegner, K. Mathias, Stoudmann, Candice, Blokesch, Melanie, Le Roux, Frédérique
• Other Authors: Agence Nationale de la Recherche, European Research Council, Howard Hughes Medical Institute, Swiss National Science Foundation
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2019
• Subjects: Pacific; Crassostrea gigas; Pacific oyster
• Online Access: http://dx.doi.org/10.1111/1462-2920.14776; https://onlinelibrary.wiley.com/doi/pdf/10.1111/1462-2920.14776; https://onlinelibrary.wiley.com/doi/full-xml/10.1111/1462-2920.14776

Summary Pacific oyster mortality syndrome affects juveniles of Crassostrea gigas oysters and threatens the sustainability of commercial and natural stocks of this species. Vibrio crassostreae ( V . crassostreae ) has been repeatedly isolated from diseased animals, and the majority of the strains have been demonstrated to be virulent for oysters. In this study, we showed that oyster farms exhibited a high prevalence of a virulence plasmid carried by V . crassostreae , while oysters, at an adult stage, were reservoirs of this virulent population. The pathogenicity of V . crassostreae depends on a novel transcriptional regulator, which activates the bidirectional promoter of a type 6 secretion system (T6SS) genes cluster. Both the T6SS and a second chromosomal virulence factor, r5 . 7 , are necessary for virulence but act independently to cause haemocyte (oyster immune cell) cytotoxicity. A phylogenetically closely related T6SS was identified in V . aestuarianus and V . tapetis , which infect adult oysters and clams respectively. We propose that haemocyte cytotoxicity is a lethality trait shared by a broad range of mollusc pathogens, and we speculate that T6SS was involved in parallel evolution of pathogen for molluscs.