RESUMO
An 84-day feeding trial was conducted to evaluate the effect of the dietary administration of Bacillus subtilis on the growth performance, body composition, intestinal probiotic recovery, hematology, and non-specific immunity of Nile tilapia (Oreochromis niloticus) raised at two stocking densities. Five hundred twenty male Nile tilapias (32.63 ± 1.25 g) were distributed in 16,800-L tanks. The experimental design was completely randomized using four replications and a 2 × 2 factorial scheme with two stocking densities (18.75 fish m(-3) 62.50 fish m(-3)) and two diets (control and with probiotic). The probiotic-supplemented diet included 5 × 10(6) CFU g feed(-1). There were no significant differences (P > 0.05) in the growth performance, body composition, and levels of cortisol and glucose between the animals fed with the control diet and the animals fed with the probiotic-supplemented diet. Differences in the growth performance were observed between the fish reared at different stocking densities; in particular, the fish raised at the high stocking density exhibited reduced weight gain, feed intake, and specific growth rate compared with those raised at the low stocking density. The B. subtilis remained viable after its inclusion in the feed, storage, and passage through the stomach, which demonstrations the feasibility of using this bacteria as a probiotic. Higher values (P < 0.05) in the plasma lysozyme levels and phagocytic activity were observed in the fish that received the probiotic-supplemented diet and reared at the high stocking density, but this difference was not observed in the fish raised at the low stocking density and fed the different diets. The administration of the probiotic caused decreases in the number of erythrocytes and the hematocrit level in the fish reared at the high stocking density, but these erythrocytes showed higher values of mean corpuscular hemoglobin. The stocking density was shown to be a stressor agent that causes a lower fish growth rate. The fish fed the control diet and raised at the high stocking density had lower lysozyme levels than the fish subjected to the other treatments. The inclusion of the probiotic bacteria B. subtilis at a concentration of 5 × 10(6) CFU g feed(-1) benefited the innate immune system of Nile tilapia by decreasing the stress associated with exposure to a high stocking density, increasing the mean corpuscular hemoglobin, and improving the innate immune system (lysozyme and phagocytic activities of macrophages).