abstract
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 m3 62.50 fish m3
) and two diets (control and with probiotic). The
probiotic-supplemented diet included 5 106 CFU g feed1
. 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 5 106 CFU g feed1 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).
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