Dietary Administration Effects of Exopolysaccharide Produced by Bacillus tequilensis PS21 Using Riceberry Broken Rice, and Soybean Meal on Growth Performance, Immunity, and Resistance to Streptococcus agalactiae of Nile tilapia (Oreochromis niloticus)

SIMPLE SUMMARY: Overuse of antibiotics has resulted in antibiotic-resistant bacteria and adverse changes in aquaculture ecology. Thus, natural bioproducts have been sourced for use in aquaculture as substitutes for antibiotics. This study investigated bacterial exopolysaccharide (EPS) as an antioxidant and a stimulant for fish immune systems. Agro-industrial biowaste carbon sources that are more economical than sugars are preferred to mitigate the production cost of EPS for commercial applications. In this study, the EPS produced from riceberry broken rice and soybean meal by Bacillus tequilensis PS21 bacteria from milk kefir grain within 72 h displayed antioxidant activity and antimicrobial activity toward pathogenic bacteria Streptococcus agalactiae. In Nile tilapia (Oreochromis niloticus), EPS supplementation at a high dose of 2.0 g EPS/kg feed significantly increased fish survival post-challenge with S. agalactiae EW1 and also boosted the fish’s immune system. The study suggested a method for utilizing agro-industrial biowaste as a value-added EPS source for a bio-circular green economy model to preserve a healthy environment while also enabling sustainable aquaculture. ABSTRACT: Overuse of antibiotics in aquaculture has generated bacterial resistance and altered the ecology. Aquacultural disease control requires an environmentally sustainable approach. Bacterial exopolysaccharides (EPSs) as bioimmunostimulants have not been extensively explored in aquaculture. This study investigated EPS produced from 5% w/v riceberry broken rice as a carbon source and 1% w/v soybean meal as a nitrogen source by Bacillus tequilensis PS21 from milk kefir grain for its immunomodulatory, antioxidant activities and resistance to pathogenic Streptococcus agalactiae in Nile tilapia (Oreochromis niloticus). The FTIR spectrum of EPS confirmed the characteristic bonds of polysaccharides, while the HPLC chromatogram of EPS displayed only the glucose monomer subunit, indicating its homopolysaccharide feature. This EPS (20 mg/mL) exhibited DPPH scavenging activity of 65.50 ± 0.31%, an FRAP value of 2.07 ± 0.04 mg FeSO(4)/g DW, and antimicrobial activity (14.17 ± 0.76 mm inhibition zone diameter) against S. agalactiae EW1 using the agar disc diffusion method. Five groups of Nile tilapia were fed diets (T1 (Control) = 0.0, T2 = 0.1, T3 = 0.2, T4 = 1.0, and T5 = 2.0 g EPS/kg diet) for 90 days. Results showed that EPS did not affect growth performances or body composition, but EPS (T4 + T5) significantly stimulated neutrophil levels and serum lysozyme activity. EPS (T5) significantly induced myeloperoxidase activity, catalase activity, and liver superoxide dismutase activity. EPS (T5) also significantly increased the survival of fish at 80.00 ± 5.77% at 14 days post-challenge with S. agalactiae EW1 compared to the control (T1) at 53.33 ± 10.00%. This study presents an efficient method for utilizing agro-industrial biowaste as a prospective source of value-added EPS via a microbial factory to produce a bio-circular green economy model that preserves a healthy environment while also promoting sustainable aquaculture.