DS0102 -

OYSTER AS A NICHE FOR VIBRIO EVOLUTION AND PATHOGEN EMERGENCE – REVENGE

Submission summary

Global change and anthropogenic activities have caused a worldwide increase in reports of vibrio-associated diseases with ecosystem-wide impacts on humans and marine animals. This is illustrated by the recent outbreaks of Crassostrea gigas diseases in France associated with vibrios. Understanding of the ecological and evolutionary dynamics of these infectious agents is important for diagnosing, predicting and preventing diseases in farmed and wild species. We recently demonstrated that during oyster mortality events, particular ecological populations of Vibrio are more abundant in oyster tissues than in surrounding seawater. Here we define an ecological population as the genetic unit representing a cohesive ecology, gene flow and social attributes. We also showed that some of these ecological populations (such as V. crassostreae) contain a high proportion of pathogenic strains. The pathogenicity of V. crassostreae relies on the presence of a large mobilizable plasmid. Interestingly a preliminary study demonstrated that this plasmid is not detected in V. crassostreae strains isolated from oysters sampled in Northern Europe where mortalities are scarce. Our current hypothesis is that V. crassostreae is an oyster commensal that turned into a pathogen after a plasmid acquisition. While V. crassostreae is abundant in all diseased oysters, this population always co-occurs with diverse populations in each individual. Experimental infections have suggested that interactions between strains may play a role in the disease. Hence, populations are the unit of pathogenesis in juvenile oysters in a polymicrobial context. Therefore, an approach aimed at specifically eliminating a target population is needed to understand its specific role in the oyster disease process. One such strategy is to utilize naturally occurring predators of bacteria as Vibriophages that are extremely abundant in nature, present high host specificity and can thus be considered as an “eco friendly” method.
A separate study suggested that co-evolution drives association of oysters and vibrios. Larvae from two genetically and geographically distinct sources (Texel and Sylt, North Europe) were exposed to diverse pathogenic vibrios isolated from both locations. For several vibrio species, sympatric combinations were associated with a lower mortality rate. The genome comparison of several strains highlighted two genes that clustered locally matching the virulence phenotype while showing incongruent patterns relative to the evolutionary history of the strains. The selective pressures leading to these geographic rather than phylogenetic matches are unknown, but it seems likely that they result from adaptations to living associated with oyster hosts.

The overall aim of this project is to investigate the evolution of vibrio virulence in relation to the oyster as a host and phages as predators. This will allow to determine whether i) pathogenic genotypes of Vibrio emerge in areas experiencing mass mortalities; ii) Vibrio and oysters co-evolve; iii) Vibrio-phage infection networks are more highly connected in co-occurring phages and hosts; iv) elimination of a pathogenic population (V. crassostreae) by a phage cocktail improves oyster survival. To this end, we will use a combination of population modeling, comparative and functional genomic analysis in addition to experimental pathology.

• In Task 1, we will explore the population structure of Vibrio in oysters and seawater in locations affected by a gradient of disease severity (Brest, Texel and Sylt) and investigate V. crassostreae diversity.
• In T2, we will perform cross infection experiments using lab-bred specific pathogen free (SPF) oysters produced from genitors collected at Brest, Texel and Sylt and Vibrios isolated at the 3 different locations.
• In T3, we will explore the differences in the phage-V. crassostreae infection network over time and space and test the effect of a phage cocktail on oyster disease.

Project coordination

Frédérique Le Roux (Laboratoire de Biologie Intégrative des Modèles Marins (LIBMM))

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

Eligo Bioscience ELIGO BIOSCIENCE
IFREMER-LEMAR Laboratoire des sciences de l'Environnement MARin (LEMAR)
CNRS Bretagne-Pays Loire-LIBMM Laboratoire de Biologie Intégrative des Modèles Marins (LIBMM)

Help of the ANR 502,532 euros
Beginning and duration of the scientific project: December 2016 - 48 Months

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