Probiotics relieve growth retardation and stress by upgrading immunity in Nile tilapia (Oreochromis niloticus) during high temperature events

Global warming is disastrous to aquatic animals and supplementation of probiotics might mitigate its adverse effects. Therefore, to mitigate the effects of high temperature on growth retardation, stress and immunity, multi-species probiotics consisting of Bacillus subtilis (5×10⁹ cfu/ml), B. thuringiencis (4×10⁹ cfu/ml), Lactobacillus plantarum (5.8×10⁹ cfu/ml), and L. buchneri (6.5×10⁹ cfu/ml) were added in rearing water (1.0 ml/L) of Nile tilapia (Oreochromis niloticus). Fingerlings (1.08 ± 0.13 g) were reared at normal (31ºC) and elevated (34ºC and 37ºC) temperatures with or without probiotics for 6 weeks in triplicates. At the end of the experimental period, weight gain (WG) and specific growth rate (SGR) decreased significantly at 37ºC in fish supplemented with or without probiotics relative to normal temperature (31ºC). Importantly, at the high temperature (37ºC), WG and SGR were improved in fish supplemented with probiotics in contrast to fish reared without probiotics. The physiological status was improved by increasing the hemoglobin level and number of red blood cells and lowering white blood cells and glucose levels in the probiotics-treated high-temperature group. Frequencies of cellular and nuclear abnormalities of erythrocytes were significantly lowered in probiotics-supplemented fish. Histological observation demonstrated that multi-probiotics mitigated the high-temperature stress through increasing mucosal fold fattening, goblet cells, and the size of lamina propria and enterocytes in the intestine. Fish under higher temperatures (34 and 37ºC) showed up-regulated expression of the stress-related heat shock protein 70 (hsp70) gene, while its expression was decreased after probiotics addition. In contrast, two antioxidant-related genes (superoxide dismutase; SOD) and catalase; CAT) showed opposite expression patterns. The expression of three immune response-related genes (tumor necrosis factor alpha; TNF-α, interleukin 1 beta; IL-1β, and interferon gamma; IFN-γ) down-regulated with increase of temperature, while their expressions were increased after probiotics addition. Therefore, addition of probiotics in aquatic environment improved hemato-biochemical properties, blood cell structure, and immunity which ultimately relieve the growth retardation and stress in extreme temperatures in Nile tilapia.