Optimizing bacillus subtilis dosage to boost growth and gut health in nile tilapia (oreochromis niloticus) in brackish water conditions

¹ Aquaculture Study Program, Faculty of Agriculture, Universitas Muhammadiyah Makassar, Makassar, Indonesia
² Aquaculture Study Program, Faculty of Agricultural, Animal, and Fisheries Sciences, Universitas Muhammadiyah Pare pare, Parepare, Indonesia
³ Fisheries Research Center for Brackish Water Aquaculture and Fisheries Counseling, South Sulawesi, Indonesia
⁴ Research Centre for Marine Aquaculture, National Research and Innovation Agency, Cibinong Science Center-Biology Building, Bogor, Indonesia
⁵ Research Center for Veterinary, National Research and Innovation Agency, Cibinong Science Center-Biology Building, Bogor, Indonesia

^* Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

Tilapia farming is central to global food security, yet efforts to expand production into saline environments are often limited by reduced growth, metabolic strain, and increased disease susceptibility. This study examined whether synbiotic diets combining Bacillus subtilis with selected prebiotic substrates could improve growth, hematology, intestinal structure, immune responses, and gut microbiota in Nile tilapia exposed to salinity stress. A 12-week feeding trial, structured using a Completely Randomized Design, tested graded levels of B. subtilis. Growth performance, feed efficiency, and survival were measured together with hematological parameters and gut histology, including villus height and mucosal thickness. Immune gene expression and microbiota profiles were assessed through RT-PCR and Next-Generation Sequencing. Synbiotic supplementation enhanced specific growth rate, feed conversion ratio, and survival, with the optimal response at 10⁵ CFU/mL. Blood indices suggested improved oxygen transport, and histology showed stronger intestinal integrity. Molecular analyses revealed greater microbial diversity and upregulated immune-related genes, offering increased resistance to Aeromonas hydrophila. These results demonstrate the promise of synbiotic nutrition in reducing salinity-related stress and strengthening tilapia health. Future work should include long-term trials and explore multi-strain or multi-prebiotic formulations to further refine synbiotic strategies for sustainable aquaculture.