Our work has provided key novel insights into the role of vibrios in Crassostreae gigas oyster infections (Bruto 2017, 2018; Parizeth 2018). Establishing specific pathogen-free oysters enabled assessing the infection processes under natural conditions (Petton 2015) and investigating the disease ecology of Vibrio populations in oyster farms affected by diverse diseases (Le Roux 2016). For example V. crassostreae is a Vibrio species particularly abundant in animals affected by the Pacific Oyster Mortality Syndrome (Bruto 2017; de Lorgeril 2018). Although members of this species are genetically diverse, most of them can cause disease. By combining experimental ecology, high throughput infection assays, comparative and functional genomics and cellular biology, we discovered diverse molecular mechanisms involved in oyster infection by V. crassostreae (Lemire 2015; Bruto 2017; Rubio 2019; Piel 2019). This encompasses an ancestral core gene, R5.7, encoding an exported protein of unknown function and a conjugative plasmid, pGV1512, expressing a type 6 secretion system (T6SS). Both the T6SS and the core gene are necessary to cause the death of the oyster immune cells referred to as the hemocytes.
In the environment, vibrios are subject to different selective pressures that can influence the evolution of virulence. The "coincidental selection" hypothesis suggests that the cytotoxicity of virulent vibrios may result from the selection of genes involved in defense against grazers. This hypothesis could explain the presence of MARTX toxin encoding genes in V. splendidus (Bruto 2018). Biotic interactions can also lead to trade-offs. For instance, acquisition of genes involved in O-antigen synthesis in a V. splendidus promotes resistance to predation by an amoeba but result in reduced virulence in oysters (Daniel 2020). Genes responsible for local adaptation of vibrios to oysters and leading to attenuate virulence have also been identified in other contexts (Wegner 2019). On the opposite, we showed in V. crassostreae that the virulence plasmid pGV1512 was selected in oyster farms, probably to counteract an increased resistance of oysters affected by the Pacific Oyster Mortality Syndrome.
Major achievements of the team. Vibrio population structure coincides with virulence. Species-specific virulence mechanisms have been identified. They converge to a samspecies with different and potentially additive virulence mechanisms. Consistent with the hypothesis of "shared weapons", experimental infections have demonstrated that some strains are moderately virulent when injected into animals individually, and display heightened virulence in mixed experimental infections.e end, the death of the hemocytes. Oysters can be infected by diverse Vibrio.
Our lab is currently involved in the DECICOMP project (Deciphering the whole complexity of the Pacific oyster mortality syndrome for modeling epidemiological risk) funded by the ANR (French National Research Agency) aimed at determining how temperature, oyster age and diet create conditions permissive- or not- to the expression of this syndrome (http://ihpe.univ-perp.fr). Within this project, our lab is specifically involved in understanding the implication of vibrios under permissive and non-permissive conditions to the disease expression and functionally characterizing the role of candidate genes in virulence.