Study of the impact of ocean warming on the expression of virulence factors in Vibrio parahaemolyticus and the response of the host Exaiptasia pallida to infection

Rising ocean temperatures lead to the emergence of pathogenic bacteria, particularly of the genus Vibrio spp. Among these, Vibrio parahaemolyticus (V. parahaemolyticus) is a pathogen of both humans and marine organisms. This bacterium, which is widespread throughout the world, is responsible for severe cases of gastroenteritis resulting from the consumption of undercooked infected seafood. At the same time, a major public health problem is emerging: ocean pollution from microplastics, which are becoming a new vector for pathogens. It is becoming increasingly important to study the effects of rising temperatures on adhesion and biofilm formation by V. parahaemolyticus on abiotic (plastic) and biotic (shellfish, etc.) surfaces, as well as the response of marine organisms to infection.The first chapter of this thesis focuses on the virulence of V. parahaemolyticus in seawater at three different temperatures: 21°C (average temperature in the Mediterranean Sea), 27°C (tropical water temperature), and 31°C (predicted for 2050 according to IPCC).The virulence mechanisms of V. parahaemolyticus are well described. We studied the initial stage of infection, adhesion to hosts or substrates via adhesins (such as MAM7 and GbpA) and biofilm production via type 4 pili (such as PilA or MSHA), followed by secretion of the toxin TDH. Our results suggest that elevated temperatures promote adhesion of bacteria to plastics via adhesins, stimulate biofilm formation via type 4 pili, and increase secretion of toxins such as TDH.In the second part of this work, we wanted to investigate the effects of global warming on the process of V. parahaemolyticus infection in a marine organism. We chose Exaiptasia pallida (E. pallida), an anemone belonging to the cnidarians, an emerging model with an innate immune system similar to that of humans and representative for the study of host-pathogen relationships. Two infection methods were used: infection by filtration (simulating a natural process) and direct injection into the animal's coelenteron, which showed similar responses. The morphological responses of E. pallida to infection were similar to those of humans, with tissue degradation and increased mucus secretion after infection.To localise Vibrio in the different tissues of the anemone, we optimised a transparentization technique (AnemoClear) that allowed us to localise the bacteria in the mesenteries. This result was confirmed by in vivo imaging and confocal imaging on fixed tissue.Mesenteric filaments are known in the literature to be involved in the immune response in Cnidaria. We focused our subsequent studies on the identification of cells infected with V. parahaemolyticus (which constitutively expresses GFP (Vp-GFP)) in these mesenteric filaments isolated after dissection. In the absence of antibodies directed against anemone-specific markers, we used their autofluorescence detected by spectral cytometry techniques. We identified a population of cells with an autofluorescence spectrum similar to that of human monocytes. This population showed intense double positivity with the spectrum of Vp-GFP grouped into vacuoles at 27°C, whereas they were scattered at 31°C.The work performed allowed us to study the response of E. pallida infected with V. parahaemolyticus at different water temperatures and to show the effects of heating on the maintenance of structures during infection. We identified cells resembling human immune cells involved in bacterial phagocytosis.