Journal Article
Selection of Vibrio crassostreae relies on a plasmid expressing a type 6 secretion system cytotoxic for host immune cells
Damien Piel; Maxime Bruto; Adèle James; Yannick Labreuche; Christophe Lambert; Adrian Janicot; Sabine Chenivesse; Bruno Petton; K. Mathias Wegner; Candice Stoudmann; Melanie Blokesch; Frédérique Le Roux
Environmental Microbiology · Vol. 22, Issue 10 · pp. 4198-4211 · 2020
Abstract
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.