Environmental reservoirs of vibrios pathogenic oysters and planktonic species affecting their dynamics in the Thau Lagoon, south of  France

Ec2co (2014-2015)
Project leader: Delphine Destoumieux-Garzόn
Strains of vibrios and a variant of the herpes virus OsHV-1 have been found recurrently in moribund oysters during episodes of C. gigas mortality during the warm months of the year. The virulence of various Vibrio strains belonging to the Splendidus group and the V. aestuarianus species has been confirmed in experimental oyster infection. However, to date, very little is known about the transmission of these strains of Vibrio to the oyster and environmental species that could be vectors. Surprisingly, under experimental infection conditions, when the oysters are immersed in seawater without plankton or suspended particles (filtered seawater), infection with these vibrios does not induce mortality. One hypothesis is that the size of the vibrios (1 μm), which is below the prey size spectrum for the oyster (10 to 500 μm), does not allow efficient filtration. Conversely, in the natural environment, bacterial strains could use planktonic species for vectors, as has been demonstrated for the human pathogen V. cholerae. In a context of mass mortality of oysters, it has appeared important to determine if certain planktonic species are able to concentrate strains of V. aestuarianus and Splendidus pathogenic for oysters and to identify the potential reservoirs of these pathogens in the oyster environment. We have set up a 2-year environmental monitoring program in the Thau lagoon, which supports significant oyster farming activity in the Mediterranean.
Main results
The results of the InterVibrio project showed the absence of V. aestuarianus in the sediment and water column of the Thau lagoon in 2013 and 2014 when significant oyster mortalities were observed. The pathogenic populations of the Splendidus Clade, V. tasmaniensis and V. crassostreae, were found in these two compartments. Sediment under oyster tables has emerged as a potential reservoir for these pathogenic populations. We also showed that the pathogenic populations of the Splendidus clade were involved close associations with planktonic species (zoo and phytoplankton) likely to favor the infection of oysters. We have finally shown that the virulence traits of these vibrios in oysters (cytotoxic factors, resistance factors to phagocytic degradation) were also major determinants of their resistance to grazing by marine protists present in the oyster environment. Thus, plankton species could not only promote the transmission of pathogens but also promote their emergence by selecting virulent genotypes in oysters.
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Partner 1 – Interactions Hôtes-Pathogènes-Environnements (UMR 5244 – Ifremer, CNRS, Université de Perpignan Via Domitia, Université de Montpellier) :  Jean-Luc Rolland, Carmen Lopez-Joven, Philippe Haffner, Guillaume M. Charrière, Aurore Poirier, Etienne Robino, Celina Abi-Khalil, Delphine Destoumieux-Garzόn
Partner 2 – MARine Biodiversity Exploitation and Conservation  (UMR MARBEC), Montpellier. Audrey Caro, Cécile Roques, Eric Abbadie, Estelle Masseret, Mohamed Laabir, Delphine Bonnet.
Partner 3 – Unité Ifremer SG2M/ LGPMM, La Tremblade : Marie-Agnès Travers