A Novel Selenite Reductase SerV01 In Intestinal Probiotics Enhanced Selenium Supplementation In Broilers

Selenium(Se)is an essential trace element that plays an important role in human health.Selenium-deficient areas account for 51% of the total area of China where have been recognized as high prevalence of selenium deficiency diseases.Because selenium shows beneficial to toxic properties in a narrow concentration range,insufficient or excessive intake can lead to adverse effects.The bioavailability of Se supplement also depends on forms of selenium.Compared to selenite and organic selenium,elemental selenium has been found to have higher antioxidant activity,bioavailability and low toxicity in recent years,which makes it a promising substitute to traditional Se supplements in animal diets.Further to obtain products that are used for human Se supplementation,such as Se-rich meat,eggs and milk.The synthesis of selenium nanoparticles(Se NPs)usually requires physical and chemical approaches due to the low content and yield in nature.However,the high temperature and pressure,acidic media,expensive chemicals used,and more toxic by-products generated during the preparation process pose a threat to the environment.Therefore,the focus should be on new methods for the synthesis of Se NPs which has advantages such as high efficiency,low cost and environmental protection.In this study,we found that SerV01,a novel selenite reductase expressed in Staphylococcus aureus LZ-01,can convert Se(IV)to Se NPs and has the ability to be used as an enhancer for the production of selenium supplements.The optimal temperature for enzymatic activity is 37 °C and the optimal p H is 8.0.In addition,intestinal microorganisms usually play an important role in the redox of substrates,the adsorption and detoxification processes of heavy metals can lead to changes in metal valence.For instance,Escherichia coli Nissle1917(EcN)has probiotic effects and is used for and many intestinal diseases,and is a good heterologous expression vector for exogenous proteins.However,there are few studies using modified probiotic EcN to reduce heavy metals for trace element supplementation in humans.In this thesis,we constructed a novel whole-cell biocatalyst that exhibits the selenite reductase SerV01 on the cell surface of EcN and colonized in the broiler intestine as a new engineered bacterium(EcN-IS).It works by directly reducing Se(IV)in the intestine to Se NPs.This study explored the effects of EcN-IS bacteria on broiler performance,immune function,antioxidant properties,tissue selenium content and intestinal flora changes.The aim is to evaluate how the change of chemical form of Se supplementation affects antioxidant capacity and state in intestinal flora.The results showed that after 7 weeks(49 days)of feeding,the EcN-IS group had a 0.07%intestinal colonization rate.At the same time,the EcN-IS and Se(IV)feed broilers with no significant difference in liver damage indicators,such as AST,ALT and MDA),but increased selenase activity(GSH-px,Trx)and immunity(Ig A)compared to the group supplemented with only 0.5 mg/kg Se(IV)broilers.The composition of the intestinal microbiota was also affected by different forms of dietary selenium supplementation.The genus-level community analysis showed that broilers colonized with EcN-IS and supplemented with Se(IV)had an increased abundance of Lactobacillus by 9.28% compared to the control,and Propionibacterium gained0.19% of total intestinal flora.It showed benefits to maintain homeostasis of the intestinal tract and further affected selenium bioavailability and selenoprotein expression in broilers.Besides,liver and chicken breast mucle of the group fed with EcN-IS and Se(IV)were enriched with more selenium,0.87 mg/kg and 0.52 mg/kg,respectively.Besides,excretion in feces of EcN-IS group was reduced to 0.79 mg/kg compared to Se(IV)group.The selenium supplementation technology used in this paper mainly combines synthetic biology with intestinal remediation and has advantages in several aspects.Firstly,the selenite added to the feed does not need to enter the cell membrane and then go out as reduction products in the form of Se NPs,which makes it more efficient in the detoxification mechanism.In addition,intestinal production instead of in vitro production of Se NPs greatly reduces the cost,and the reduction products Se NPs can be directly delivered to the intestine and absorbed by both intercellular and cytosolic means.Inorganic selenium must first bind to organic ligands in the intestine before being absorbed by the organism,and organic selenium absorption is through the pathway of amino acid metabolism that raises the Se content in muscle.Therefore,the technical route and management system which bases on synthetic biology to achieve therapeutic intestinal health has high cost performance and is worthy of further research and application in the future.This new method provides a theoretical basis for the discovery of novel selenite reductases and its use in poultry farming.Livestock meat products can act as a diet for subsequent practical application of selenium supplementation in humans.