Effect of dietary Lactococcus lactis and Bacillus subtilis on the innate immunity, intestinal microbiota, histometrical indices, and resistance against Aeromonas hydrophila in Oscar, Astronotus ocellatus Agassiz, 1831

This work aimed to investigate the effect of dietary Lactococcus lactis and Bacillus subtilis on the immune responses, intestinal microbiota, and resistance to pathogens of Oscar, Astronotus ocellatus. During 70 days trial, 300 juveniles (8.96±0.033 g) were fed diets enriched with L. lactis and B. subtilis. The treatments included 150, 300, 450 mg kg^-1 of dietary L. lactis (LL₁₅₀, LL₃₀₀, LL₄₅₀); 150, 300, 450 mg kg^-1 of dietary B. subtilis (BS₁₅₀, BS₃₀₀, BS₄₅₀); 150, 300, 450 mg kg^-1 of diet an equal mixture of L. lactis and B. subtilis (MIX₁₅₀, MIX₃₀₀, MIX₄₅₀); and a non-supplemented control group. At the end of the rearing period, histological, immunological, and intestinal microbiota indices in treatments were investigated. To evaluate disease resistance, 15 fish in each treatment were infected in each treatment by Aeromonas hydrophila. The results showed that adding B. subtilis and L. lactis, particularly in MIX₃₀₀, reduced the anaerobic heterotrophic bacterial microbiota and increased lactic acid bacteria (LAB) in fish. The highest white blood cell (WBC) level was recorded in the LL₁₅₀ group. The lymphocytes in fish fed LL₁₅₀, LL₃₀₀, MIX₁₅₀, and MIX₃₀₀ diets were changed and neutrophils of LL₁₅₀, LL₃₀₀, LL₄₅₀, MIX_300, and MIX₄₅₀ were significantly increased. Monocytes in fish fed MIX₃₀₀ and MIX₄₅₀ diets raised significantly. The IgM, ACH₅₀, and lysozyme levels in fish-fed diets enriched by bacteria, especially in LL₄₅₀, were significantly higher than the control treatment. The intestinal villi in LL₄₅₀, BS₁₅₀, and MIX₄₅₀ were significantly higher, showing lower damages than the other treatments. The survival rates of the infected fishes were higher in MIX₁₅₀ and MIX₃₀₀ groups.