Bacillus spp. in Fish Farming: Enhancing Fish Performance, Health and Farming Practices

Aquaculture has become a crucial component of global food and nutrition security, offering a sustainable alternative to meet the growing demand for seafood while alleviating pressure on wild fish populations. Among various strategies to enhance aquaculture productivity, probiotics have gained considerable attention for their ability to improve fish growth performance, overall health and environmental farming conditions. This review explores the potential of Bacillus spp. as probiotics in fish farming, with particular emphasis on B. subtilis, B. licheniformis, B. amyloliquefaciens, B. velezensis, B. pumilus, B. coagulans and B. megaterium. Evidence indicates that Bacillus spp. can enhance fish growth performance by improving feed efficiency, promoting gut health and innate immune responses and contributing to improved water quality through enhanced nutrient cycling and reduced environmental pollutants. Among these species, B. subtilis emerges as the most extensively studied and applied, likely due to its robustness, abundance, safety and multifunctional probiotic properties. In contrast, other Bacillus spp. remain relatively underexplored, despite their potential to provide complementary or strain-specific benefits. This review also highlights key knowledge gaps, including the limited understanding of probiotic mechanisms at the metabolomic level. Future research integrating multi-omics approaches and evaluating both single- and multi-strain Bacillus probiotics will be essential for optimising probiotic design and advancing sustainable aquaculture practices.