Study On The Formation Mechanism Of Desirable Properties Of Streptococcus Thermophilus Based On Omics

Streptococcus thermophilus is a common starter culture that is widely used in the dairy industry.It is often used in the production of health care products since some S.thermophilus strains can produce active substances that are beneficial to the host.With the improvement of people's living standards,consumers have higher requirements for fermented products.The currently used S.thermophilus strains can no longer fully satisfy these needs.It is therefore essential to screen novel S.thermophilus strains for desirable probiotic traits.Finding desirable S.thermophilus strains and gaining insights into their genetic backgrounds can not only meet the production requirements of the fermentation industry,but also help to enrich the diversity of natural S.thermophilus resources.The characteristics of S.thermophilus strains including growth capacity,acidification rate,carbohydrate metabolism efficiency,and the survival rate in host gastrointestinal tracts,etc.are closely related to the quality,taste and commercial value of the fermented product.The possession of desirable properties is an important criteria for screening novel strains for commercial value.This study was conducted using the 22 S.thermophilus strains previously isolated from traditional Chinese fermented dairy products.These strains were tested regarding their resilience to stress,and their capacity to metabolize carbohydrates.Combined with previously measured growth abilities,acidification rates,exopolysaccharides(EPS)yields etc,S.thermophilus CS5,CS9,CS18 and CS20 had the most desirable traits.In this study,the complete genomes of S.thermophilus CS5,CS9,CS18,and CS20 were sequenced,and contrasted with the existing complete genome sequences of S.thermophilus.The results showed that the gene distribution is not necessarily related to its growth environment,and also revealed the horizontal gene transfer between S.thermophilus strains.For use in the production of food and health products,the strains must be studied at the gene level and must be proven safe.Through evaluation of the virulence factors,antibiotic resistance,transposable factors,hemolysis and the presence of endogenous plasmids,the four strains CS5,CS9,CS18,and CS20were found to be safe.The phenotype of desirable S.thermophilus strains can be affected and regulated at the gene level.In this study,the genomes of S.thermophilus CS5,CS9,CS18 and CS20were analyzed to determine the glycolysis pathways,differences in the CRISPR/Cas immune system,and the ability to produce active substances.The results showed that the CS9 genome contains the gene gad encoding glutamate decarboxylase,and its GABA production was up to 950.36 mg/L.In-vitro simulation experiments have shown that the glutamate decarboxylase system is an effective acid-tolerant mechanism in S.thermophilus cells.The proteolytic capacity of S.thermophilus strains is closely related to its acidification rate.In this study,fluorescence quantitative PCR was used to analyze the gene expression of the proteolytic system of S.thermophilus CS5,CS9,CS18 and CS20from the transcription level.The results showed that,in addition to the prt S gene(encoding serine protease),the S.thermophilus proteolytic system is also regulated by other genes involved in the proteolytic activity,two-component signal transduction system(TCS)genes,and the transcriptional regulator Cod Y.In Prt S~-strain CS9,the synergistic regulation is particularly significant.In addition,this study shows that TCS of S.thermophilus positively regulates its proteolytic capacity.S.thermophilus strains that produce EPS can be used not only in fermented food but also in health products.Increasing the EPS yield of S.thermophilus can not only improve the flavor and texture of fermented dairy products,but also help improve human health,thereby increasing its commercial value.In this study,S.thermophilus CS18 showed good growth ability,outstanding physiological traits,and an EPS yield of up to 178.2 mg/L when cultured in M17.Based on transcriptome analysis,the effect of different carbohydrates including glucose,sucrose,galactose,and mannose on the EPS production of S.thermophilus and its transcriptional regulation was determined.The results suggested that EPS production and monosaccharide composition are directly affected by the eps gene cluster.This can help provide a theoretical and experimental basis for the screening and modification of high-EPS-yielding thermophilic strains.