From Spore Biology to System Performance: Strain-Resolved Mechanisms, Biosafety and Translational Readiness of Bacillus Probiotics in Aquaculture.

Bacillus-based probiotics are widely promoted as antibiotic-sparing interventions in aquaculture, yet translation from controlled trials to commercial production remains constrained by strain heterogeneity, inconsistent dosing, incomplete biosafety assessment and frequent inference of mechanisms from non-specific biomarkers. This scoping review reorganizes the evidence reported for 67 empirical studies published during 2015-2025 into a strain-host-route-system framework. Across finfish and aquatic invertebrates, Bacillus supplementation was commonly associated with improved growth, feed utilization, digestive-enzyme activity, mucosal or systemic innate immune indices, challenge survival and, in water-applied or biofloc settings, nitrogenous-waste control. The evidence nevertheless does not support a universal optimal dose: reported regimens span colony-forming units per gram or kilogram of feed, colony-forming units per millilitre of culture water and mass-based commercial formulations, and efficacy varies with strain identity, host developmental stage, delivery route, diet, salinity, temperature and challenge design. Mechanistic support is strongest for pathogen antagonism, enzyme provision, barrier modulation and environmental bioremediation, whereas claims concerning stable colonization, antiviral action and antimicrobial-resistance mitigation are often indirect. Bacillus should therefore be evaluated at strain rather than genus level, with whole-genome screening for virulence determinants, toxins, transferable resistance genes and mobile elements. We propose a translational-readiness framework integrating identity, potency, formulation stability, host compatibility, environmental persistence, biosafety and farm-scale validation. This reframing positions Bacillus probiotics as promising but conditional tools for antibiotic stewardship and One Health-aligned aquaculture.