Effects Of Different Nitrogen Sources On The Phenotypic Characteristics Of Exopolysaccharide Of Streptococcus Thermophilus And Its Omics Analysis

Exopolysaccharide(EPS)of lactic acid bacteria is a natural green thickener,emulsifier and stabilizer,which can improve the quality and taste of dairy products.Currently,it is one of the hot spots in the field of dairy products research.This paper takes Streptococcus thermophilus IMAU 20561 isolated from traditional fermented dairy products as the research object,to study the phenotypic characteristics of exopolysaccharide produced by this strain in different nitrogen sources.and combines genomics and transcriptomics to explore the differences in the expression of genes related to the biosynthesis of exopolysaccharide particles at different growth stages.The main research results are as follows:1.S.thermophilus IMAU 20561 was inoculated into different nitrogen source mediums,and the production of exopolysaccharide was measured by the phenol-sulfuric acid method.When the soybean peptone was the single nitrogen source,the amount of exopolysaccharide produced by the strain was 480.7 mg/L.When casein peptone is used as single nitrogen source,the strain's EPS has the largest molecular weight.There are significant differences in the structure of EPS which obtained from different nitrogen sources.The EPS obtained from soybean medium is mainly composed of glucuronic acid,glucose and galactose.The EPS obtained from micro tryptone medium is mainly composed of glucuronic acid,glucose and galactose.The EPS obtained from casein peptone medium and basic medium M17 have similar monosaccharide composition,which is mainly composed of mannose,glucose and galactose.2.The genome length of S.thermophilus IMAU 20561 is 1716258 bp,and the GC content accounts for 39.03% of the whole genome.A total of 1914 CDS genes were annotated,including a gene cluster of exopolysaccharide with a length of 22.3 kb.The gene cluster of exopolysaccharide contains highly conserved genes eps A,eps B,eps C,eps D,The genes encoding glycosyltransferases are eps E,eps F,eps H,gene 0919,gene0914,gene 0911.The genes eps O and eps X related to the transfer of exopolysaccharides between the membrane and the peptidoglycan layer are annotated downstream of Orf14.9.3.Transcriptomics analysis of S.thermophilus IMAU 20561 in different growth stages on different nitrogen source media.The GO function annotation results showed that the differential genes were significantly enriched in amino acid biosynthesis and metabolism,ribonucleotide biosynthesis and metabolism,IMP biosynthesis and metabolism,and phosphorus metabolism processes,etc.The KEGG function annotation results showed that the differential genes were enriched in amino acid biosynthesis,glycolysis,phosphotransferase system,fructose and mannose metabolism,etc.4.Bases on the transcriptomics analysis,that was found that the production of exopolysaccharide is highest when soybean peptone is the nitrogen source,the expression of some genes closely related to the biosynthesis of exopolysaccharide,such as eps A,eps B,and genes encoding glucose phosphate mutant enzyme,UDP-glucose4-epimerase and UTP-1-phosphate glucouroyltransferase,were all up-regulated.This discovery indicates that the expression of these genes may directly affect the production of exopolysaccharide.