The Probiotic Effects Of Host-associated Bacillus Velezensis In Diets For Hybrid Yellow Catfish(Pelteobagrus Fulvidraco♀×Pelteobagrus Vachelli♂)

This study was to evaluate the potential of a host-associated Bacillus velezensis as a probiotic for hybrid yellow catfish（Pelteobagrus fulvidraco♀×Pelteobagrus vachelli♂）.The diets（B0 to B5）contained with 0,0.90×10⁸,0.80×10⁹,0.85×10¹⁰,0.90×10¹¹,0.83×10¹²CFU/kg B.velezensis YFI-E109 were fed to the fish with initial weight 3.07±0.08 g in a recirculating aquaculture system for six weeks with three replicates,respectively.The effects of probiotics were analyzed according to the growth performance,body composition,liver and intestinal morphology,intestinal microflora and the liver metabolism.The results showed that:（1）B.velezensis YFI-E109 was demonstrated to exert excellent resistance to Edwardsiella tarda,Aeromonas hydrophila,Aeromonas caviae and Aeromonas veronii.（2）B.velezensis genome analysis showed that most abundant genes of B.velezensis YFI-E109 were distributed in carbohydrate and amino acid metabolism,and most of the KEGG pathway was enriched in the metabolism-related pathway.（3）Compared with the results of aquaculture experiments,the fish in B3 and B4 group had better growth performance,higher intestine amylase（AMS）and lipase activities（LPS）compared with other groups（P<0.05）.With the increase of the supplement of B.velezensis YFI-E109,the content of crude protein and crude lipid in the whole fish of hybrid yellow catfish increased significantly.The addition of B.velezensis YFI-E109 reduced the content of triglyceride（TG）in the serum of hybrid yellow catfish,and decreased glucose（GLU）which in the serum of B4and B5 groups significantly.Compared with others,the hybrid yellow fish of B3 group had higher albumin（ALB）in the serum and had lower malondialdehyde（MDA）in liver.It was shown significantly increased in hybird yellowfish of B1 and B2 groups that activity of catalase（CAT）（P<0.05）.（4）The obvious damage in the liver and intestinal was observed in the fish of B0 and B5 groups.The muscular thickness of the intestinal was significantly increased,and a large number of liver cells showed cellular vacuoles and nuclear migration.But these statuses were improved of fish in B3 group.（5）The abundance of Mycoplasma,Ralstonia and Actinobacter in the intestine of hybrid yellow catfish in B3 and B5 groups decreased significantly.（6）Comparing the results of liver differential metabolites and related differential pathways,there were 19 kinds of differential metabolites up-regulated and 9 kinds of differential metabolites down-regulated in B3 VS B0;41 differential metabolites up and 21 differential metabolites down in B5 VS B0.Most of these differential metabolites are enriched in lipid metabolism,amino acid metabolism,carbohydrate metabolism and nucleotide metabolism.In conclusion,dietary B.velezensis YFI-E109 supplementation has the potential to improve growth,liver metabolism,and liver and gut health,reshaping the gut microbiome of hybrid yellow catfish.Excessive B.velezensis YFI-E109 would reduce the prebiotic effects.The recommended dietary supplementation of B.velezensis YFI-E109 is 0.31×10¹⁰to 0.77×10¹¹CFU/kg for hybrid yellow catfish according to the quadratic regression method by plotting specific growth rate,MDA and activities of amylase against dietary B.velezensis YFI-E109 levels.