| Summary: | Bacteria of the genus Vibrio are ubiquitous in aquatic environments. They adopt free and associated lifestyles. They establish mutualistic, commensal and parasitic symbioses with numerous metazoa. In the healthy Crassostrea gigas oyster, an abundant and diverse microbiota that includes many Vibrio is maintained under immune homeostasis. However, under the influence of biotic or abiotic stressors a dysbiosis is created favoring the proliferation of opportunistic Vibrio species, which can induce the death of oysters. This occurs during the Pacific Oyster Mortality Syndrome (POMS), which is caused by an immunosuppressive virus, OsHV-1 µvar. We have here characterized the Vibrio / C. gigas interactions in health and disease. We show that in populations of V. splendidus associated with healthy oysters, virulence and colonization capacity can be constrained by the selection of O-antigen structures that are more easily recognized by the host immune system but which confer resistance to grazing by marine amoebae. This trade-off between the ability to colonize its host and environmental persistence is likely the cause of the great structural diversity observed for the O-antigen in this species. In a pathological context, we have shown that the species associated with the oyster are for the most part cytotoxic for the immune cells of their host, regardless of the environment considered (Atlantic, Mediterranean). This cytotoxicity is a key determinant of the escape from the oyster's powerful cellular defenses and determines the infectious success. We observed that the molecular bases of cytotoxicity are specific to each Vibrio species studied. Finally, beyond opportunistic pathogens that use cytotoxicity (V. tasmaniensis, V. crassostreae, V. splendidus, V. harveyi), we have identified simple opportunists, such as V. rotiferianus, which not only benefit from the immunosuppressive activity of other pathogens but also adopt strategies of non-cooperative acquisition of public goods, such as siderophores produced by the ...
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