** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** DISEASE OUTBREAKS ARE A MAJOR IMPEDIMENT TO RELIABLE AQUACULTURE PRODUCTION. OUR PRIOR AFRI FUNDING HAS LED TO THE DEVELOPMENT, SCIENTIFIC VALIDATION, AND NOVEL FORMULATION OF A PROBIOTIC BACTERIUM,PHAEOBACTER INHIBENSS4, THAT IS NOW COMMERCIALLY AVAILABLE FOR OYSTER LARVICULTURE. S4 TREATMENTS IN HATCHERIES PROTECT AGAINST PROMINENT PATHOGENS, INCLUDINGVIBRIO CORALLIILYTICUSRE22.RECENT RESULTS SUGGEST THAT S4 HAS BROADER PROBIOTIC ACTIVITY AGAINST BACTERIAL AQUACULTURE DISEASES. OUR DATA ALSO REVEAL THAT RE22 KILLS LARVAE USING TWO PREVIOUSLY UNRECOGNIZED VIRULENCE MECHANISMS, TYPE VI AND TYPE III SECRETION SYSTEMS, AND THAT S4 DOWNREGULATES THESE PATHWAYS.OUR GOAL IS TO DEVELOP NOVEL LOW-COST AND 'GREEN' TOOLS FOR AQUACULTURE. THIS PROJECT WILL TEST THE HYPOTHESIS THAT SPECIFIC MOLECULAR MECHANISMS EMPLOYED BY THE PROBIONT AND PATHOGEN DETERMINE THE OUTCOMES OF INTERSPECIES INTERACTIONS AND, ULTIMATELY, THE SURVIVAL OF THE HOST. IN OBJECTIVES 1 AND 2, WE ADDRESS KNOWLEDGE GAPS REGARDING THE MOLECULAR MECHANISMS USED BY THE PATHOGEN RE22 TO KILL HOSTS. USING A GENOMICS AND BIOINFORMATICS DRIVEN APPROACH, WE WILL DISSECT THE TYPE VI AND TYPE III SECRETION SYSTEMS OF RE22 TO REVEAL THE CRITICAL COMPONENTS OF THESE VIRULENCE NANOMACHINES. MUTAGENESIS WILL BE USED TO VERIFY THE ROLES OF SPECIFIC GENES. IN OBJECTIVE 3, WE WILL USE AN INNOVATIVE TRANSCRIPTOMICS DRIVEN APPROACH TO TEST HOW PROBIONT S4 MODULATES PATHOGENESIS PATHWAYS IN OTHER AQUACULTURE PATHOGENS. THIS PROJECT ADDRESSES THE PROGRAM PRIORITY AREAS OF (1) DISEASE PREVENTION, (2) THERAPEUTIC INTERVENTIONS FOR DISEASE REDUCTION, AND (3) CREATING NEW KNOWLEDGE IN THE CELLULAR, MOLECULAR, AND GENOMIC/GENETIC ASPECTS OF HEALTH AND DISEASE.