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Some medicinal plants have been known as immunostimulants, and the medicinal plants extract has been used to control the outbreak of the disease in aquaculture for many years. In this study, a total of 270 crucian carp (30 ± 5 g) were randomly distributed in 9 aquaria (55 cm l × 40 cm w × 50 cm h) and divided into three feeding groups including 0 (Control), 50 mg kg -1 (Diet A) and 100 mg kg -1 (Diet B) of salidroside. The expression of immune-related genes (IL-1β, TNFα, MYD88, CXCL-8, TGF-β, and IL-11) in the kidney had a significant increase when the crucian carp fed with Diet B for 4 weeks (P < 0.05). Meanwhile, the expression of IL-1β, TNFα, and CXCL-8 in the spleen was significantly up-regulated when the fish fed with Diet B (P < 0.05). Higher serum alkaline phosphatase (AKP) activity, catalase (CAT) activity, superoxide dismutase (SOD) activity, and complement C3 content were found in the fish which fed with salidroside-supplemented diet. Our results also proved that fish fed with salidroside-supplemented diet for four weeks, especially at a concentration of 100 mg kg -1 diet, improved the protection of crucian carp against A. hydrophila. The amount of A. hydrophila in the kidney and spleen was significantly decreased in salidroside-supplemented diet groups (P < 0.05). In conclusion, the present results demonstrate that the addition of salidroside for four weeks can improve the immune response of crucian carp and increase the protection against the pathogen, especially at the concentration of 100 mg kg -1 diet. The protective effect of the salidroside to the crucian carp could be used as alternatives to antibiotics for controlling fish diseases in aquaculture., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

The application of antibiotics in aquaculture poses a huge hidden danger to aquatic products and water environment safety. Thus, the aim of this study was to isolate probiotics that can replace antibiotics and antagonize Aeromonas hydrophila from the gut of the grass carp, and then explore the main antibacterial substances of this bacteria. Pseudomonas monteilii JK-1 was isolated from the gut of grass carp and showed significant inhibition on A. hydrophila. The safety assessment revealed that the isolated P. monteilii JK-1 was not toxic to grass carp and did not exbibit hemolytic activity on sheep blood agar plates. Disc-diffusion assays using a panel of antibiotics showed that JK-1 was susceptible to selected antibiotics. In vitro study indicated that strain JK-1 can survive at pH 6.5–7.5, and tolerate bile up to 0.5% concentration. The trial test showed dietary supplementation of JK-1 significantly improved the resistance of grass carp against A. hydrophila , and the cumulative survival rate increased by 40% compared with the control group. 1-Hydroxyphenazine isolated from the metabolite of P. monteilii JK-1, had a good effect in against A. hydrophila in vitro and in vivo. The minimum inhibitory concentration of 1-hydroxyphenazine against A. hydrophila was 31.25 μg/mL. In vitro antimicrobial assays of 1-hydroxyphenazine showed that 1-hydroxyphenazine at 100, 250 μg/mL inhibited the growth of A. hydrophila after 6 and 4 h, respectively. Grass carp inoculated with A. hydrophila and treated by intraperitoneal application of 1-hydroxyphenazine had a survival rate of 75% compared with that of untreated fish (35%, 30%). These results indicate the protective effect of the P. monteilii JK-1 and 1-hydroxyphenazine against A. hydrophila could be used as alternatives to antibiotics for controlling fish diseases in aquaculture. • Pseudomonas monteilii JK-1 was isolated from the gut of grass carp and had the ability to against Aeromonas hydrophila. • The antibacterial compounds, named 1-hydroxyphenazine, was separated from JK-1 strain. • Pseudomonas monteilii JK-1 administered in diet protected grass carp against A. hydrophila infection. • Intraperitoneal injection administered with 1-hydroxyphenazine could protected grass carp against A. hydrophila infection. • Pseudomonas monteilii JK-1 can be developed into a preparation for the alternatives of antibiotics in aquaculture. [ABSTRACT FROM AUTHOR]