Development and evaluation of a formulation of probiontPhaeobacter inhibens S4 for the management of vibriosis inbivalve hatcheries

Larval eastern oysters (Crassostrea virginica) grown in shellfish hatcheries are suscep-tible to bacterial diseases, particularly vibriosis. Probiotics are microbes that conferhealth benefits to the host and have been identified as promising tools to manage dis-eases in aquaculture. The marine bacterium Phaeobacter inhibens S4 (S4) protects larvaleastern oysters against challenge with the bacterial pathogen Vibrio coralliilyticus RE22(RE22). A concentrated liquid formulation of probiont S4 that maintained high cell via-bility after long-term storage was developed for commercial use in shellfish hatcheries.The safety and efficacy of the formulation were tested in six different trials in twohatcheries. The S4 formulation was added to C. virginica larvae culture tanks daily at10 4 colony forming units (CFU)/mL from Day 1 post fertilisation until Day 6, 12 or 14,depending on the trial. Treatment of larvae in the hatchery with the S4 formulation didnot significantly affect the survival and growth of the larvae. Formulated probiont S4treatment in the hatchery led to a significant increase in relative percent survival (RPS)when larvae were subsequently challenged with the pathogen RE22 (105 CFU/mL) for24 h in a laboratory challenge as compared to probiotic-untreated RE22-challengedlarvae (RPS increase of 46%–74%, p < 0.05). These results suggest that this novel S4 formulation is a safe, easy-to-use and effective tool in preventing larval losses due tovibriosis in hatcheries.