Etude des infections à vibrionaceae chez les larves de mollusques bivalves à partir d'un modèle larve de Pecten maximus

The control of diseases remains a question of major importance in order to reduce pathological risks in bivalve larval rearings. For such a purpose, the knowledge of diseases, pathogens and routes of infection is required. A new species of vibrio pathogenic to P. maximus larvae, Vibrio pectenicida, was described by phenotypic and genotypic characters. It belongs to the V. splendidus group, where sorne other new pathogenic species were already found. The pathogenesis of V. pectenicida was studied by histological methods and experimental exposures in conventional and axenic larval rearings. These studies assessed the pathogenic character of V. pectenicida strains and the lack of particular symptoms during the disease. The location ofbacteria in the tissues can be explained by a translocation phenomenon. The toxic activity of the pathogenic vibrio seems to be due to bacterial internai factors. In fact, vibrio bacterial lysats, mainly cytoplasmic extracts, exhibited a lethal activity in axenic larvae. To confinn the occurence of these toxic internai factors, a chemiluminescent (CL) test with adult hemocytes was perfected and used. The pathogenic vibrios inhibited the CL activity of hemocytes from bivalve adult. The inhibitory effect, observed with intact bacteria, was found again in the cytoplasmic fraction. This fact confinned the existence of internai toxic factor(s). According to the partial purification, one of them may be a small molecule (below 3 kDa), hydrophilic, protease and acid resistant and thenno-stable. This toxin was different ofpreviously described toxins. The relationships between the immunity of adult, very resistant to bacterial infection, and the larvae were investigated by two experiments: reproduction of disease in P. maximus adults by a strain related to V. splendidus and the injection of V. pectenicida to adults inducing a decrease of the hemocytes activity. Sorne applications were proposed by the end of this work to avoid the use of antibiotics, detrimental to the environment, during the frrst steps of the mollusc production.