Dietary Yucca schidigera and Bacillus subtilis enhance growth and immunity while reducing ammonia discharge in Nile tilapia (Oreochromis niloticus)

This study aimed to evaluate the effects of dietary inclusion of Yucca schidigera extract and Bacillus subtilis on the growth performance, immune status, and water quality of Nile tilapia (Oreochromis niloticus) fingerlings. A total of 120 fish (13.54 ± 0.32 g) were randomly distributed into four groups (three replicates each). Each group fed one of the four experimental diets for 65 days: a control diet without additives (CON), a yucca-supplemented diet (0.2 g kg⁻¹; Y), a Bacillus-supplemented diet (1.0 g kg⁻¹; B), or a combined yucca and Bacillus diet (0.2 + 1.0 g kg⁻¹; YB). Water quality analysis indicated that unionized ammonia (NH₃) concentration was the highest in CON and the lowest in YB (p ≤ 0.05). B and YB exhibited superior growth performance and feed efficiency compared to Y and CON (p ≤ 0.05). The highest activities of protease, amylase, and lipase were recorded in YB, followed by B and then Y, while CON exhibited the poorest results. Phagocytic activity, respiratory burst, and complement activity were the greatest in YB, intermediate in B and Y, and the lowest in CON (p ≤ 0.05). Lysozyme activity was found to be significantly higher in B, followed by YB, with the lowest activity observed in CON (p ≤ 0.05). Antioxidant enzyme levels (superoxide dismutase, glutathione peroxidase, reduced glutathione, and total antioxidant capacity) were maximized in YB and minimized in CON with intermediate results in B (p ≤ 0.05), whereas malondialdehyde concentration showed the opposite trend. YB and B exhibited higher levels of interleukin-8 and tumor necrosis factor alpha compared to Y, while CON showed the lowest expression (p ≤ 0.05). Interleukin-1 beta expression was the highest in B, followed by YB, then Y, with the lowest levels observed in CON (p ≤ 0.05). Thus, dietary supplementation with Yucca schidigera extract and Bacillus subtilis, in combination, may represent an effective strategy to improve water quality, growth performance, and immune-antioxidant responses in Nile tilapia.