Evaluation Of Yeast Hydrolysate And Bacillus Subtilis As Feed Additives In Largemouth Bass Micropterus Salmoides

Two experiments were conducted to study the effects of yeast hydrolysate and Bacillus subtilis on growth,immunity,antioxidant,liver and intestinal health of largemouth bass,respectively,and further evaluate the feasibility of these two additives application in largemouth bass feed.1.Effects of dietary yeast hydrolysate supplementation on the growth,digestive enzyme activities,antioxidant capacity and liver morphology of largemouth bass Micropterus salmoidesFive isonitrogenous and isolipid diets were formulated by adding 0(control,CON),0.94(YH-1),1.88(YH-2),2.82(YH-3)and 3.76 g/kg(YH-4)yeast hydrolysate in the diets to feed largemouth bass juveniles(LMB,initial mean body weight,17.0±1.32 g).Each diet was assigned to three replicate tanks with 20 fish per tank,and the culture experiment lasted for 8 weeks.The results showed that the growth,feed utilization and survival rate of LMB were not sensitive to dietary yeast hydrolysate(P>0.05).When yeast hydrolysate supplemental level was above 1.88 g/kg,VSI and HSI were significantly lower than those of the CON fish(P<0.05).Whole body composition was not impacted by yeast hydrolysate(P>0.05),while higher liver crude protein content was recorded when yeast hydrolysate supplemental level was more than 1.88 g/kg(P<0.05).Yeast hydrolysate could improve liver function of LMB,such as serum AST activity,and there was a significant difference in the YH-4 fish(P<0.05).Liver morphology results showed that liver vacuoles were significantly reduced when the yeast hydrolysate supplemental level was exceeded 1.88 g/kg,indicating that the yeast hydrolysate could improve the liver health of LMB.Liver GSH content in the YH-4 fish was significantly higher than that in other treatments(P<0.05),while T-AOC,CAT,SOD and GST activities were not impacted by yeast hydrolysate(P>0.05).In addition,intestinal antioxidant indicators(CAT,SOD,T-AOC,GST and GSH)and digestive enzyme activities(amylase,lipase and trypsin)were not impacted by yeast hydrolysate(P>0.05).In conclusion,dietary yeast hydrolysate dose not promote the growth performance,intestinal antioxidant and digestive enzyme activities,but is beneficial to the improvement of liver morphology and antioxidant capacity of LMB.It is recommended that the content of dietary yeast hydrolysate in LMB diet should be higher than 1.88 g/kg,and the optimal supplemental level should be further studied.2.Effects of dietary Bacillus subtilis DSM 32315 supplementation on the growth,immunity,and intestinal morphology and microecology of juvenile largemouth bass Micropterus salmoidesThree isonitrogenous and isolipid diets were formulated by adding 0(control,CON),1.5(BS1.5,3×10~6CFU/g feed)and 3 g/kg(BS3,6×10~6 CFU/g feed)B.subtilis DSM 32315 in the basal diets to feed LMB(initial mean body weight,8.32±0.06 g).Each diet was assigned to four replicate tanks with 20 fish per tank,and the culture experiment lasted for 9 weeks.The results showed that the growth and feed utilization of LMB were not sensitive to dietary modifications(P>0.05).Compared with the CON fish,dietary BS supplementation did not affect the hematological parameters(WBC and RBC counts,and HCT value)of LMB(P>0.05),but the BS1.5 fish performed better than the BS3 fish.Serum innate immunity indicators(LZM activity,and C3 and C4 levels)were improved with dietary BS supplementation at both doses,while higher serum Ig M level was only recorded at 1.5 g/kg feed(P<0.05).All the sampled fish exhibited intact intestines,but the intestinal architecture(villus height and muscular layer thickness)still benefited from dietary BS inclusion at both doses(P<0.05).Although intestinal anti-inflammatory response(m RNA levels of il-10 and tgf-β1)did not differ between the BS supplemented fish(P>0.05),the BS1.5 fish obtained superior pro-inflammatory response(m RNA levels of il-1β,il-8 and tnf-α)over the BS3 fish(P<0.05).Compared with the CON fish,the abundance of the harmful Plesiomonas Shigelloides decreased in the gut microbiota of the BS1.5 fish(P<0.05).Despite the BS3 fish obtained higher diversity of gut microbiota,the abundances of potential beneficial(Cetobacterium somerae and Shewanella xiamenensis)and harmful bacteria(Acinetobacter johnsonii and P.Shigelloides)increased or decreased simultaneously(P<0.05).Taken together,it was concluded that dietary BS DSM 32315inclusion presented some positive effects on LMB.Although the BS1.5 fish performed better than the BS3 fish,the best supplemental dose need to be further studied.