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, MIX300, 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.