| Salmonids aquaculture industry has become the mainstay of cold water fish in markets worldwide.The salmonids are rich in essential amino acid and unsaturated fatty acid,which can effectively reduce the cholesterol in the blood,so as to prevent from cardiovascular disease actively caused by arteriosclerosis.Salmonids are high-quality aquatic products which are very popular.Recirculting aquaculture of salmonids is the most promising way to meet the market requirement.RAS was characterized by high density,high quality and low pollution.Aquaculture environmental degradation,water pollution,fish disease and many other issues restrict the rapid development of salmonids aquaculture.Efficient disease prevention strategies were urgent.Strain screening of probiotic bacteria was considered to be a promosing way to improve the fish health,enhance the ability of resistance to pathogens,which can be a sustainable ecological priority strategy of salmonids aquaculture.The present study was to assess the effects of a potential probiotic Bacillus velezensis V4 and Rhodotorula mucilaginosa on growth,immune response,antioxidant capability,disease resistance and intestinal microbiota of juvenile salmonids reared in a recirculating aquaculture system.This studies provide strong theoretical basis for commercialization and application of this bacteria.The main results are summarized as follows:1.In order to understand the differences in the intestinal microbiota of Atlantic salmon with different health statuses,we sampled the intestinal content of healthy and unhealthy Atlantic salmon(Salmo salar L.),the ambient water of unhealthy fish,and the biofilter material in the recirculating aquaculture system(RAS).Denatured gradient gel electrophoresis(DGGE)was used to scan the intestinal mictobiota of fish intestine.The V4–V5 regions of the prokaryotic 16 S r RNA genes in the samples were further analyzed by Mi Seq high-throughput sequencing.Representative members of the intestinal microbiota were identified.The intestinal microbiota of the healthy fish included Proteobacteria(44.33%),Actinobacteria(17.89%),Bacteroidetes(15.25%),and Firmicutes(9.11%),among which the families Micrococcaceae and Oxalobacteraceae and genera Sphingomonas,Streptomyces,Pedobacter,Janthinobacterium,Burkholderia,and Balneimonas were most abundant.Proteobacteria(70.46%),Bacteroidetes(7.59%),and Firmicutes(7.55%)dominated the microbiota of unhealthy fish,and Chloroflexi(2.71%),and Aliivibrio and Vibrio as well as genera in the family Aeromonadaceae were most strongly represented.Overall,the intestinal hindgut microbiota differed between healthy and unhealthy fish.Much more accurate diseased fish model was built to study the relationship between healthy and diseased fish infected by Aeromonas salmonicida.The species of intestinal microbitota were significantly decreased by the infection of A.salmonicida,and also the intestinal bacterial diversity.Serious microecological imbalance in intestinal microbiota was caused by the A.salmonicida infection.2.To assess the effects of probiotic compound of Bacillus velezensis V4 CGMCC 10149 and Rhodotorula mucilaginosa CGMCC 1013 on the growth performance and gut microbiota of rainbow trout(Oncorhynchus mykiss).Rainbow trout were reared in recirculating aquaculture system(RAS)and fed with four experimental diets which contained compound probiotics of 0/0 CFU g-1(Control,B.velezensis V4/ R.mucilaginosa),5 × 106 / 5 × 107 CFU/g(T1),1.5 × 107 /1.5 × 108 CFU/g(T3),2.5 × 107 / 2.5 × 108 CFU/g(T5)for 40 days,respectively.Weight gain ratio,feed conversion ratio and specific growth rate were significantly enhanced in diet T1 at the end of the trial.Survival rate was significantly improved in the probiotics treatments,especially in diet T3.The gut microbiota of the rainbow trout was assessed by Mi Seq sequencing of the V4 region of 16 S r RNA genes.Interestingly,no obvious deviations were observed in the whole gut microbial community structures of the different treatments according to unweighted pair-group method with arithmetic mean(UPGMA)and principal coordinate analysis(PCo A).Compared with the control,the mean Chao1 estimator,Shannon-Wiener index,and Simpson index were higher in the probiotic compound treatments.In the gut of rainbow trout fed with the probiotic compound diets,the relative abundance of probiotic Psychrobacter,Sporolactobacillus,Lactococcus and Streptomyces were increased,while the potential pathogens such as Burkholderia were lower than those of control.3.A feeding trial of 62 days was conducted to investigate the effects of dietary administration of a probiotic compound consisting of B.velezensis V4 and R.mucilaginosa on growth,immune response,antioxidant capability,disease resistance and intestinal microbiota of juvenile Atlantic salmon(Salmo salar L.)reared in a recirculating aquaculture system(RAS).Administration of the probiotic compound had significant effects on growth performance and feed utilization.Mortality was also significantly reduced(P < 0.05).The level of malondialdehyde(MDA),GPT(glutamic pyruvic transaminase)and NO(nitric oxide)was decreased by the addition of probiotic compound in feeding trial.T-SOD(total superoxide dismutase),GSH(glutathione),GSH-Px(glutathione peroxidase),CAT(catalase),ACP(acid phosphatase),T-AOC(total antioxidant capacity)and Ig M was enhanced(P < 0.05)in T1 group.Cumulative mortality was also reduced in the probiotic diets.The t-SOD,CAT,GOT,GPT and ACP were increased by the infection of A.salmonicida challenge,especially 48 h after challenge.Immune Lysozyme,Ig M and cortisol was also elevated.This results demonstrated B.velezensis V4 and R.mucilaginosa compound as a beneficial dietary probiotics for juvenile Atlantic salmon reared in a RAS with the capability of improving growth performance and enhancing disease resistance by immunomodulation.No obvious deviations were observed in the whole gut microbial community structures of the different treatments.4.Furthermore,a 70-day systematic laboratory experiment was designed to explore the effect of dietary administration of single B.velezensis V4 on growth,survival rate,immune response,antioxidant capability,disease resistance and intestinal microbiota of juvenile salmonids.The results represented that survival rate and feed conversion ratio was elevated by the addition of B.velezensis V4.No obvious weight gain was found.Strong immune response,antioxidant capability and disease resistance was found,especially after the challenge of A.salmonicida.The micro-ecological balance of intestinal microbiota of rainbow trout was less affected.The relative abundance of Bacillus cereus,Bacillus methylotrophicus and Lactobacillus murinus were accelerated by the addition of B.velezensis V4,especially for Bacillus methylotrophicus. |
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