The recent expansion in the aquaculture of bivalves (oysters, scallops, clams) has been constrained by the impact of infectious diseases on different stages of production. At the hatchery stage, b acterial diseases such as vibriosis can lead to rapid and significant losses of larvae. The goal of our research is to apply knowledge on microbial-microbial interactions to manage the impact of bacterial diseases in shellfish hatcheries, using the bacterial pathogen Vibrio coralliilyticus RE22 (RE22) and the marine bacterium and probiont candidate Phaeobacter inhibens S4 (S4) as a model system.
Pretreatment of eastern oyster, Crassostrea virginica, larvae with probiont S4 leads to significantly increased larval survival after challenge with pathogen RE22 as compared to non-treated larvae. Genome sequencing of pathogen RE22 and probiont S4 have led to the identification of putative virulence factors in RE22 and mechanisms of action of S4 against RE22. Targeted gene knock-out experiments in RE22 showed that one of the two copies of the Type 6 Secretion System (T6SS) identified in the genome of RE22 targets mainly the larval host, while the other copy targets mainly other bacteria. T he mechanisms of action of probiont S4 are complex, including, but not limited to, antibiosis, biofil m formation, and quorum quenching. Gene expression patterns of RE22 and S4 in competition suggest that S4 can reduce expression of a variety of virulence factors in RE22, including T6SS and motility . Moreover, treatment of oyster larvae with probiont S4 leads to immunostimulation , while treatment of larvae with pathogen RE22 leads to immunosuppression. A liquid formulation of probiont S4 has been developed for commercial use in shellfish hatcheries . Daily d elivery of probiont S4 to C. virginica larvae in the hatchery led to changes in the bacterial communities in the larvae, including changes in the composition , but not relative abundance, of Vibrionaceae .
Our research contributes to better understanding of the mechanisms of virulence of pathogen RE22, as well as pathogen-probiont interactions in vitro and in vivo. The ultimate goal is to optimize probiotic use in shellfish hatcheries.