Efficient Preparation Of Selenium-enriched Candida Utilis Under Moderate Acid Stress And Its Application In Broilers Feeding

Candida utilis, a kind of yeast certificated by Food and Drug Administration of the U.S.A(FDA), can be used in food industry and medical health care. Owing to its high capability to assimilate selenium and high ability to transform inorganic selenium into organic selenium, C. utilis is suitable for preparation selenium-enriched yeast by transforming inorganic selenium. Selenium-enriched C. utilis contains high levels of protein, fat, B-Vitamins, glutathione(GSH) and other valuable ingredients, hence, a broader application scope of this Se-yeast can be expected.During the microbial preparation of selenium-enriched C. utilis, the contents of intracellular organic selenium and GSH are both important indexes to determine the performance of the selenium-enriched yeast. In this thesis, aiming at high organic selenium content and high intracellular GSH content, batch culture of C. utilis under moderate acid stress was carried out. The mechanism of moderate acid stress improved the performance of selenium-enriched yeast was also explored in molecular and physiological aspects. Finally, the influence of selenium-enriched C. utilis on the antioxidant capacity and immune performance of broilers was investigated. The main results of this thesis are presented as follows.(1) The capacities of selenium enrichment and glutathione biosynthesis as well as antioxidant properties of a sodium selenite-resisting C. utilis Se-1301 were investigated in flasks and a 5 L stirred fermentor. The results illustrated that the highest intracellular organic selenium and glutathione contents of 1.08 mg/g and 18.48 mg/g, respectively, were obtained under a moderate acid stress environment(pH 3.5). After assaying the activities of intracellular γ-glutamylcysteine synthetase, catalase and superoxide dismutase, together with determining the intracellular malondialdehyde content, the moderate acid stress was found to be not only favored to improve the ability of glutathione biosynthesis, but also helped to increase the antioxidant capacity of the yeast.(2) The total mRNA from C. utilis cells prepared under different culture conditions(pH5.5, pH5.5 with sodium selenite enrichment, pH 3.5 with sodium selenite enrichment) was identified and analyzed by transcriptomic approach. The results indicated that the enrichment of sodium selenite increased the expression of many genes involved in metabolic and transporting processes. The proteins encoded by these genes were found to be focused on various sub-cellular organelles and cell membrane, which improved the biotransformation of organic selenium and also improved the enrichment of intracellular selenium. In addition, moderate acid stress can improve the primary metabolism, large molecules metabolism and nitrogen metabolism. Specially, the organic selenium and GSH biosynthesis metabolism, as well as the gene expression related to ATP synthesis was further improved, which in turn increased the biosynthesis of GSH and the accumulation of intracellular organic selenium.(3) The application of selenium-enriched C. utilis in broilers feeding was studied. The influence of selenium-enriched C. utilis supplemented in the feed on the antioxidant capacity and immune response of broilers was studied by determining the growth performance of broilers, selenium contents in blood and tissues, the activities of glutathione peroxidase(GSH-Px), catalase(CAT) and superoxide dismutase(SOD), the contents of MDA and GSH, the levels of immunoglobulin A(IgA), immunoglobulin G(IgG) and immunoglobulin M(IgM) in serum, and the immune organ index. Results indicated that selenium from different resources had no significant effect on the broilers’ average daily gain(P>0.05), compared with the control group without the addition of selenium in feed, thus it cannot significantly improve the broilers’ growth performance. The levels of IgA, IgG and IgM in serum of the experimental groups showed higher results than those of the control. The activities of SOD, CAT and GSH-Px in blood, and the serum selenium levels are significantly higher than that of the control group. The MDA level is significantly lower than that of the control group. According to all the above results, it can be concluded that the selenium-enriched C. utilis showed much more advantages than other additives supplemented into the feed of broilers to increase both antioxidant capacity and immune response.