Effects of dietary supplementation of Bacillus subtilis DSM 32315 on growth, immune response and acute ammonia stress tolerance of Nile tilapia (Oreochromis niloticus) fed with high or low protein diets

Aquatic animals have benefited from Bacillus subtilis-based probiotics over the past few decades. This study evaluated the effects of B. subtilis DSM 32315 probiotics as a feed additive on growth, immune response and resistance to acute ammonia challenge in Nile tilapia. Specifically, four supplemental levels (0%, 0.1%, 0.2%, and 0.3%) of B. subtilis probiotics were tested under two dietary protein levels (32% and 28%). Five replicate tanks were randomly allotted to each dietary treatment, with each tank containing 30 Nile tilapia. After 8 weeks of feeding, Nile tilapia in each tank were exposed to 43.61 mg/L of total ammonia nitrogen for 48 h. The results revealed that reducing protein levels from 32% to 28% did not affect growth performance or antioxidant capacity. However, the low protein diet tended to induce an inflammatory effect shown by increased expressions of TGF-β and IFN-γ genes (P < 0.05) in the liver. The impact was alleviated by the probiotic supplementation. Compared with the non-supplemented group, 0.1% probiotic supplementation remarkably increased plasma lysozyme activity, total antioxidant capacity and complement C3 and interleukin-10 mRNA levels (P < 0.05) in the 28% protein diet, while higher supplementation of probiotics (0.3%) was shown to be beneficial for the high protein diet (32%). In both the dietary protein levels, 0.1% supplementation of probiotics promoted the antioxidant capacity of Nile tilapia before exposure to ammonia stress but higher probiotic supplementation (0.3%) proved to be necessary under ammonia stress as evidenced by higher fish survival rate. Results exhibited that supplementation with B. subtilis probiotics had a better effect on the intestinal morphology (villi height and width) regardless of protein levels. In conclusion, dietary supplementation of B. subtilis DSM 32315 probiotics at 0.1% in the low protein diet and up to 0.3% in the high protein diet showed beneficial effects on the growth, immunity, and antioxidant capacity of Nile tilapia. Under ammonia stress conditions, the higher supplementation of B. subtilis DSM 32315 probiotics at 0.3% improves stress tolerance of Nile tilapia despite the two dietary protein levels (32%; 28%).