Effects Of Dietary Bacillus Velezensis LSG2-5 On Growth Performance,Antioxidant Capacity And Immunity Of Rhynchocypris Lagowskii Dybowski

As a feed additive,probiotic Bacillus can promote the growth and enhance disease resistance of fish.However,there are currently fewer types of probiotic Bacillus additives from fish in aquaculture.Therefore,one of the main ways to solve this problem is to screen fish-source probiotic Bacillus and verify its effect.To investigate whether Bacillus velezensis from Rhynchocypris lagowskii Dybowski had a positive effect on the host,B.velezensis LSG2-5isolated from the intestinal tract of R.lagowskii was used as a feed additive and R.lagowskii(Initial weight:9.98±0.05g)was used as a study object in the study.The supplemental levels of B.velezensis LSG2-5 were CK(0 CFU/g),B-6(1.0×10~6CFU/g),B-7(1.0×10~7CFU/g),B-8(1.0×10~8CFU/g)and B-9(1.0×10~9CFU/g),respectively.The effects of B.velezensis LSG2-5 on growth performance,digestive enzyme activities,intestinal flora structure,antioxidant capacity,immunity and resistent Aeromonas hydrophila infection of R.lagowskii were investigated after56 d culture experiment and 14 d challenge experiment,and the probiotic effects of B.velezensis LSG2-5 on R.lagowskii were verified.The study consists of three parts,and the results were as follows:(1)Effects of B.velezensis LSG2-5 on growth performance of R.lagowskiiThe results showed that:At the 28th d,compared with the control group,there were no significant differences in weight gain rate,specific growth rate,feed conversion ratio,hepatosomatic index and intestinal length index in treatment groups(P>0.05).The condition factor of R.lagowskii group B-7 was significantly higher than that of control group(P<0.05).The viscera indexes of B-8 and B-9 groups were significantly higher those that of control group(P<0.05).Intestinal body indexes in B-7,B-8 and B-9 groups were significantly higher than those in control group(P<0.05).Compared with control group,the activities of trypsin,amylase and alkaline phosphatase in hepatopancreas of B-7 group were significantly increased(P<0.05).The activities of digestive and absorption enzymes(Try,AMY,γ-GT,Na~+-K~+-ATPase and AKP)in the intestinal tract of R.lagowskii were significantly increased(P<0.05).At the 56th d,the final weight,weight gain rate and specific growth rate in B-7,B-8 and B-9groups were significantly higher than those in control group(P<0.05).Feed conversion ratio of treatment groups was significantly lower than that of control group(P<0.05).The hepatosomatic indexes in B-8 group was significantly higher than that in control group(P<0.05).B.velezensis had no significant effects on viscera index,condition factor,intestinal length index and intestinal body index(P>0.05).Compared with the control group,the activities of digestive and absorbing enzymes in the hepatopancreas and intestinal tract of the treatment groups were significantly increased(P<0.05).The crude protein content in muscle of B-7 and B-8 groups was significantly higher than that of control group(P<0.05).The results of the intestinal flora structure of R.lagowskii showed that:Compared with the control group,there was no significant difference in the Alpha index of intestinal microorganisms(P>0.05).The main microbial composition in the intestinal tract of R.lagowskii is Proteobacteria,Firmicutes,Bacteroidetes and Fusobacteria.The abundance of Proteobacteria,Bacteroidetes and Fusobacteria increased first and then decreased,while the abundance of Firmicutes showed an opposite trend.According to parabolic regression analysis,the recommended dose of B.velezensis LSG2-5 was 5.00×10~8CFU/g.(2)Effects of B.velezensis LSG2-5 on antioxidant capacity of R.lagowskiiThe results showed that:At the 28th d,compared with the control group,the activities of CAT and T-AOC in hepatopancreas of B-8 group were significantly increased(P<0.05).The contents of GSH,CAT,GSH-Px and SOD activities in intestinal tract of the treatment groups were significantly increased(P<0.05).The content of MDA in the intestinal tract of R.lagowskii was significantly decreased(P<0.05).At the 56th d,compared with control group,the contents of GSH and the activities of CAT,GSH-Px,SOD,T-AOC in hepatopancreas and intestinal tract of treatment groups were significantly increased(P<0.05).The content of MDA in hepatopancreas and intestinal tract of R.lagowskii was significantly decreased(P<0.05).The relative expression levels of CAT mRNA,GSH-Px mRNA and CuZn-SOD mRNA in hepatopancreas and intestines of R.lagowskii of B-7,B-8 and B-9 groups were significantly upregulated by B.velezensis(P<0.05).At the same time,the relative expressions of CAT mRNA,GSH-Px mRNA and CuZn-SOD mRNA in the muscle of R.lagowskii of B-7 and B-8groups were significantly increased(P<0.05).(3)Effects of B.velezensis LSG2-5 on immunity of R.lagowskiiThe results showed that:At the 28 th d,LZM and IgM contents in serum of B-7,B-8 and B-9 groups were significantly higher than those in control group(P<0.05).The ACP activity in serum of B-7 group was significantly higher than that of control group(P<0.05).At the 56th d,LZM and IgM contents in serum of all treatment groups were significantly higher than those of control group(P<0.05).The C3 content and AKP activity in serum of B-8 and B-9 groups were significantly higher than those in control group(P<0.05).The C4 content in serum of B-8group was significantly higher than that in control group(P<0.05).The ACP activity in serum of B-7 group was significantly higher than that of control group(P<0.05).The relative expression levels of TGF-βmRNA and IL-10 mRNA in hepatopancreas of R.lagowskii in B-8group were significantly higher than those in control group(P<0.05).The relative expression levels of IL-8 mRNA,IL-1βmRNA and TNF-αmRNA in hepatopancreas of R.lagowskii of B-8and B-9 groups were significantly lower than those in control group(P<0.05).Compared with the control group,the expression levels of TGF-βmRNA and IL-10 mRNA in the intestinal tract of R.lagowskii showed a significant upward trend(P<0.05).The expression levels of IL-8mRNA,IL-1βmRNA and TNF-αmRNA in the intestinal tract of R.lagowskii showed a significant downward trend(P<0.05).The expression levels of TGF-βmRNA and IL-10mRNA in muscle of R.lagowskii of B-7 and B-8 groups were significantly higher than those in control group(P<0.05).The relative expression levels of IL-8 mRNA,IL-1βmRNA and TNF-αmRNA in muscle of R.lagowskii of B-7,B-8 and B-9 groups were significantly lower than those in control group(P<0.05).At the 14th day after infection with A.hydrophila,the survival rates of treatment groups were significantly higher than that of control group(P<0.05),and the survival rate of B-8 group was the highest.The contents of LZM,C3 and IgM in serum of treatment groups were significantly higher than those in control group(P<0.05).The content of C4 in serum of B-7,B-8 and B-9 groups was significantly higher than that of control group(P<0.05),and the content of C4 in B-6 group also showed an increasing trend,but there was no significant difference(P>0.05).The activities of ACP and AKP in serum of four treatment groups were significantly higher than those of control group(P<0.05).In conclusion,the supplementation of 1.0×10~7～1.0×10~9CFU/g of B.velezensis LSG2-5isolated from the intestinal tract of R.lagowskii could be promoted growth performance,the antioxidant capacity and immunity,and enhanced the disease resistance of R.lagowskii.Comprehensive consideration,the recommended dose of B.velezensis LSG2-5 in the diet was5.00×10~8CFU/g under the experimental conditions.