Larval eastern oysters (Crassostrea virginica) grown in shellfish hatcheries are susceptible to bacterial diseases, particularly vibriosis. Probiotics are microbes that confer health benefits to the host and have been identified as promising tools to manage diseases in aquaculture. The marine bacterium Phaeobacter inhibens S4 (S4) protects larval eastern oysters against challenge with the bacterial pathogen Vibrio coralliilyticus RE22 (RE22). A concentrated liquid formulation of probiont S4 that maintained high cell viability 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 two hatcheries. The S4 formulation was added to C. virginica larvae culture tanks daily at 104 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 did not significantly affect the survival and growth of the larvae. Formulated probiont S4 treatment 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) for 24 h in a laboratory challenge as compared to probiotic‐untreated RE22‐challenged larvae (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 to vibriosis in hatcheries.

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Larval eastern oysters (Crassostrea virginica) grown in shellfish hatcheries are susceptible to bacterial diseases, particularly vibriosis. Probiotics are microbes that confer health benefits to the host and have been identified as promising tools to manage diseases in aquaculture. The marine bacteriumPhaeobacter inhibensS4 (S4) protects larval eastern oysters against challenge with the bacterial pathogenVibrio coralliilyticusRE22 (RE22). A concentrated liquid formulation of probiont S4 that maintained high cell viability after long-term storage was developed for commercial use in shellfish hatcheries. The safety and efficacy of the formulation was tested in six different trials in two hatcheries. The S4 formulation was added toC. virginicalarvae culture tanks daily at 104colony forming units (CFU)/mL from day 1 post fertilization until day 6, 12, or 14, depending on the trial. Treatment of larvae in the hatchery with the S4 formulation did not significantly affect the survival and growth of the larvae. Formulated probiont S4 treatment in the hatchery led to a significant increase in Relative Percent Survival (RPS) when larvae were subsequently challenged with the pathogen RE22 (105CFU/mL) for 24 hours in a laboratory challenge, as compared to probiotic-untreated RE22-challenged larvae (Relative Percent Survival 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 to vibriosis in hatcheries.

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