| Summary: | Probiotics, “living organisms, which when administered in adequate amounts confer a health benefit to the host”, can contribute to a more sustainable aquaculture. Their administration through the diet or raising water can modulate the host immune status, improving their resistance towards infection. The antioxidant defence system of the organism is strongly related to immune system and previous studies reported enhancement in antioxidant status of shrimps and fish after probiotic administration, contributing to enhanced resistance towards infections. Nevertheless the information on oxidative stress parameters after probiotic administration in fish is still limited. The present work evaluates the effects of dietary probiotics supplementation on innate immune and oxidative stress parameters in juvenile Nile tilapia (<i>Oreochromis niloticus</i>). A standard diet (27.5% CP, 8.2% CL, 19.6MJ.kg-1 DM) was supplemented with a commercial multi-species probiotic (<i>Bacillus</i> sp., <i>Pedicoccus</i> sp., <i>Enterococcus</i> sp., <i>Lactobacillus</i> sp.) at two concentrations: A1 (3 g.kg-1; 9 × 105 CFU.g-1) and A2 (6 g.kg-1; 2 × 106 CFU.g-1) and tested against an unsupplemented diet (A0). Fish (12.8 g) were hand-fed the experimental diets (3 tanks/treatment; 20 animals per tank), 3 times a day, until visual satiation for 8 weeks. Animals were reared at 24ºC in a closed recirculating freshwater system. During the experiment, at 2, 4 and 8 weeks, blood, head-kidney and liver were sampled to study the following immunological and oxidative stress parameters: plasma lysozyme and alternative complement pathway activity (expressed as ACH50), respiratory burst activity and nitric oxide production of head-kidney leucocytes and liver lipid peroxidation (LPO), catalase (CAT), total glutathione (TG), glutathione peroxidase (GPX), and glutathione reductase (GR) activities. Respiratory burst activity and nitric oxide production in head-kidney leucocytes were not significantly affected by probiotic treatment and the same was observed regarding plasmatic lysozyme activity. However ACH50 levels were significantly higher in fish fed the A2 diet (38.3 ± 15.3 U ml-1) when compared to fish fed A0 diet (24.7 ± 9.7 U ml-1) after the 8 weeks of feeding. The present data shows that dietary supplementation of multi-species probiotics induce some degree of immune stimulation, in a time and dose dependent manner. The oxidative stress status in liver, currently under analysis, will provide a more comprehensive outlook of probiotics effects.
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