Scallop larvae resistant to a pathogenic Vibrio harbor host-associated bacteria with probiotic potential

The breeding of the scallop Argopecten purpuratus is threatened by mass mortality events mostly caused by bacterial pathogens of the genus Vibrio affecting larvae and spat cultivated in hatcheries. A strong genetic basis for the resistance of A. purpuratus larvae to vibriosis has been demonstrated, which is associated with an efficient energy metabolism and immunocompetence. However, various factors, including the microbiota composition, have a notable impact on the outcome of an infection in numerous animal models, affecting their resistance. In this context, we aimed to explore the possible contribution of host-associated bacteria to the resistance of scallop larvae to Vibrio bivalvicida VPAP30. To achieve this, we isolated culturable bacteria from four scallop families displaying contrasted resistance to VPAP30 infection. We taxonomically assigned 75 bacterial isolates by 16S rDNA sequencing, and we found that 34 isolates were exclusively associated with resistant larvae. Then, 8 isolates were pre-selected as potential probiotics based on their exclusivity to the resistant families and bibliographic records of their use. Results highlighted three isolates displaying potential, belonging to the genera Psychrobacter, Hydrogenophaga, and Shewanella. The three isolates displayed in vitro antagonistic activity against VPAP30 and a protective effect on scallop veliger larvae against infection. We observed three main aspects of this protective effect: scallop larvae pretreated with each of the three probiotic candidates showed (i) no mortality after 24 h infection, (ii) an earlier induction of immune pattern recognition receptors, and (iii) no increase in total Vibrio spp. counts after infection with VPAP30. The findings demonstrated that scallop larvae resistant to vibriosis carry distinct bacteria possessing probiotic properties suitable for the rearing of scallop larvae. This study reinforces the significance of host resistance-related microorganisms as a potential source for discovering novel endogenous probiotics with biotechnological applications.