Abstract
Recently, the frequency and severity of marine diseases have increased in association with global changes, andmollusks of economic interest are particularly concerned. A striking example of a devastating disease is the Pacific Oyster Mortality Syndrome(POMS) caused by the Ostreid Herpesvirus-1 μVar (OsHV-1 μVar) that emerged in 2008 and which heavily impacts Crassostrea gigasproduction worldwide by affecting juvenile oysters. Adult oysters are also affected by infectious diseases, especially those caused by thebacterial pathogen Vibrio aestuarianus.In my PhD, I investigated the possibility of using the beneficial effect of the oyster microbiota to fightagainst these infectious diseases. It has been reported that some bacterial strains are preferentially associated with oysters with better survivalcapacity to the POMS disease. In addition, previous work has shown that it is possible to educate the oyster immune system through an exposureto healthy microbiota during their larval development. This experiment had been achieved by using oyster donors of microbiota. This microbialeducation had the advantage of using all the potential of the whole microorganism diversity, but the transferred community were not fullycontrolled and could be hazardous. This made this whole microbiota exposure not applicable in aquaculture. In this sense, I decided to focus mywork on controlled microbial environment and investigated their potential beneficial effect in oyster health.I generated a collection of bacterialspecies from naturally disease-resistant C. gigas collected in the field, and I characterized their effects on oysters. I investigated two possiblebeneficial strategies: (1) I performed a microbial education by adding multi-strain bacterial mixes during larval development and investigated thelong-term effect of this exposure at juvenile and adult stages when oysters were challenged with OsHV-1 and V. aestuarianus. (2) I identifiedantimicrobial-producing bacterial strains and investigated their short-term effect by adding them to juvenile and adult oysters just before pathogenchallenges. Overall, I showed that both strategies can help fighting against oyster infectious diseases but strongly relies on oyster genetic.Thesefindings open new avenues for the development of microbiota-targeted prophylactic approaches to mitigate diseases in oyster farming.