Effects of selenium-enriched Bacillus pumilus on the growth performance, blood biochemical parameters, innate immune response, and oxidative activity Nile tilapia (Oreochromis niloticus) fed fishmeal-free diets

The present study is the first step towards optimizing Nile tilapia (Oreochromis niloticus) performance through a fishmeal-free diet with selenium forms (Se) and Se-enriched Bacillus pumilus. Consequently, the present interest is to study their effect on immune response and growth performance that are directly affected by hemato-biochemical indices along with analyses of intestinal histomorphology and other hepatic antioxidant enzymes in Nile tilapia. The study utilized a 2 × 3 factorial feeding design involving 450 tilapia fingerlings, each with an average body weight of 3.23 ± 0.09 g. The fish were distributed into 18 cylindrical plastic tanks, each with a volume of 0.5 m³ , containing 25 fish per tank. Six experimental diets were formulated, all of which were isonitrogenous (30 % crude protein) and isoenergetic. The diets included: A supplement-free control diet (T1), a diet supplemented with B. pumilus (T2, at concentration of 1.85 ×10⁵ CFU kg⁻¹), a diet containing 0.4 mg/kg selenomethionine (T3, O Se), a diet that was O Se-enriched with B. pumilus (T4), a diet with 0.4 mg/kg sodium selenite (T5, I Se), and a diet that was I Se-enriched with B. pumilus (T6). The outcome of the feeding experiment indicates that the inclusion of Se or B. pumilus significantly improved weight gain, final body weight, protein efficiency ratio (PER), average daily gain (ADG), specific growth rate (SGR), and feed intake in fish compared to supplement-free diet, with the highest growth indices and the best FCR recorded in organic selenium-enriched diets. The intestinal histomorphometry including intestinal villous width, absorption surface area, and goblet cells was enhanced when tilapia receiving diet fortified with selenium. The best results were observed in the organic selenium-supplemented diet. Also, diet fortified with selenium exhibited significantly raised hematocrit, leucocyte, monocytes, and lymphocytes compared to supplement-free diet. The B. pumilus enriched selenium forms and B. pumilus significantly affected these indices, with the highest Hb in group fed B. pumilus. Moreover, a notable decline was detected in aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels as opposed to a diet without supplementation in response to the addition of Se forms or B. pumilus (P < 0.05). Also, B. pumilus enriched-O Se significantly enhanced serum globulin, total protein, and albumin levels. While selenium supplementation considerable decreased lipid profiles in fish, particularly those fed a diet supplemented with organic selenium. Inorganic selenium supplementation led to the lowest glucose levels, while the highest was in supplement-free diet. Fish fed selenium forms or B. pumilus showed increased hepatic antioxidant activities, particularly in a diet enriched with organic selenium, while malondialdehyde (MDA) and nitric oxide (NO) concentrations decreased. There was an increase in IgM and C4 levels in fish that consumed a B. pumilus diet. From this, it can be derived that O Se-enriched B. pumilusis the best choice for dietary supplementation for enhancing growth, immune response, intestinal histomorphology, hepatic antioxidant enzymes, and hemato-biochemical indices in Nile tilapia.