Study of Dietary Emodin on Immune Defense in Megalobrama amblycephala against Aeromonas hydrophila

Simple Summary This study focuses on the Wuchang bream (Megalobrama amblycephala) as the research subject, aiming to investigate the effects of emodin on the total bacterial count of Aeromonas hydrophila and the immune response in various tissues of Wuchang bream following infection. The experimental diets were made by supplementing emodin at 0, 30, 100, and 150 mg kg−1 to a basal (control) diet, respectively, and fed to fish with an initial weight of 50.4 ± 2.35 g. All fish were divided into five experimental groups: uninfected fish fed with basal control diet (negative control, NC), infected fish fed with the diet supplemented with 0 (positive control group, PC), 30 (30), 100 (100), and 150 mg/kg (150) of emodin. Except for the negative control group, all other groups were injected with A. hydrophila at a concentration of 1 × 106 CFU/mL. The experiment was conducted over a period of 14 days, with sampling at various time points. The results of the experiment demonstrated that the total bacterial count of Aeromonas in the kidney, blood, and liver tissues of infected Wuchang bream was significantly affected by the dosage of added emodin and the feeding duration. Additionally, the immune response of Wuchang bream following intraperitoneal infection with A. hydrophila was also significantly influenced by emodin (p < 0.05). In conclusion, the addition of 100 mg/kg of emodin to the diet could enhance the Wuchang bream’s resistance to A. hydrophila infection by reducing the total bacterial count of pathogenic bacteria in tissues, increasing the activity of relevant immune enzymes and promoting the secretion of cytokines. This study provides a theoretical basis for practical production. Abstract This experiment aimed to investigate the effects of emodin on the total bacterial count and immune response in various tissues of Wuchang bream infected with A. hydrophila. The experimental diets were made by supplementing emodin at 0, 30, 100, and 150 mg kg−1 to basal (control) diet, respectively, and fed to fish with an initial weight of 50.4 ± 2.35 g. All fish were divided into five experimental groups: uninfected fish fed with basal control diet (negative control, NC), infected fish fed with the diet supplemented with 0 (positive control group, PC), 30 (30), 100 (100), and 150 mg/kg (150) of emodin. The fish were reared for 14 days and sampled at different time points. The results showed that the total bacterial count in the kidney, blood, and liver tissues of Wuchang bream infected with A. hydrophila was significantly affected by the supplementation and feeding time of emodin. At the beginning of the experiment, the difference in total bacterial count among the groups was not significant. On day 1, the total bacterial count in all groups was significantly higher (p < 0.05) than that in the negative control group. On day 4, the total bacterial count in all the emodin groups was significantly reduced, and the best bactericidal effect was observed in the 100 mg kg−1 group. In addition, emodin had a significant effect on the immune response of Wuchang bream after infection with A. hydrophila (p < 0.05). Compared with the other groups, the respiratory burst activity, tumor necrosis factor-α (TNF-α), interleukin-1 (IL-1) content, and white blood cell count (WBC) in the 100 and 150 mg kg−1 groups could be restored to normal levels in the shortest time (p < 0.05). Furthermore, this study also measured the complement alternative pathway activity (ACH50), plasma superoxide dismutase (SOD) activity, and malondialdehyde (MDA) content of the fish. The results showed that supplying 100 mg kg−1 emodin to the diet could significantly (p < 0.05) increase the ACH50 activity of the fish. Compared with the positive control (PC) group, the addition of emodin to the diet can inhibit the decrease in SOD activity and the increase in MDA content in the plasma of infected Wuchang bream. In conclusion, supplying 100 mg kg−1 emodin to the diet can enhance the ability of Wuchang bream to resist A. hydrophila infection by reducing the total bacterial count in tissues, increasing the activity of related immune enzymes, and promoting the secretion of cytokines. This provides a theoretical basis for production practice.