Study On The Fermentation And Metabolic Characteristics Of Streptococcus Thermophilus S4 In Milk

As one of the basic starters of yogurt,Streptococcus thermophilus has been widely used in various fermented dairy products.Streptococcus thermophilus S4 was isolated from Qula in Maqu County,Gansu Province.It has attracted people's attention due to its strong acid production ability,fast rate of fermentation and excellent flavor.And it can be used as a candidate strain for yogurt starter.However,most of the current researches on lactic acid bacteria are focus on the probiotic effects,and the intrinsic fermentation and metabolism characteristics in milk are still unclear.Thus,this study regard Streptococcus thermophilus S4 as the research object,and collected fermented milk samples in the pre-fermentation phase(p H=6.39),mid-fermentation phase(p H=5.12)and end-fermentation phase(p H=4.53).Finally,combined transcriptomics,proteomics and metabolomics technology to comprehensively explore the fermentation metabolism characteristics in milk of Streptococcus thermophilus S4.The main results are as follows:(1)Transcriptomics results showed that there were 467 differentially expressed genes in the mid-fermentation phase compared with the pre-fermentation phase in Streptococcus thermophilus S4,of which 193 were up-regulated and 274 were down-regulated;The end-fermentation phase compared with the mid-fermentation phase,there were a total of975 differentially expressed genes.497 were up-regulated and 478 were down-regulated.These differentially expressed genes are mainly regulated by carbohydrate metabolism,amino acid transport and metabolism,and nucleotide metabolism,etc.(2)Proteomics results showed that there were 126 differentially expressed proteins in the mid-fermentation phase compared with the pre-fermentation phase in Streptococcus thermophilus S4,of which 2 were up-regulated and 124 were down-regulated;The end-fermentation phase compared with the mid-fermentation phase,there were a total of 11 differentially expressed proteins.9 were up-regulated and 2 were down-regulated;It can be seen that most of the differentially expressed proteins have the highest expression levels in the pre-fermentation phase.These differentially expressed proteins are significantly enriched in aromatic amino acid biosynthesis,purine metabolism,glycolysis/gluconeogenesis,glutamate metabolism,branched-chain amino acid metabolism and some other pathways.(3)Metabonomics results showed that compared with pre-fermentation phase,10 different metabolites were identified in the mid-fermentation phase in Streptococcus thermophilus S4,including glycinamide nucleotides,pyridoxamine,phenylalanine,etc.The end-fermentation phase compared with the mid-fermentation phase,15 different metabolites were identified,including L-glutamine,L-asparagine,tyrosine,tryptophan,etc.These significantly differential metabolites most enriched in amino acid metabolism pathway.Further analysis revealed that the pathways related to amino acid metabolism were the most active.These pathways not only provide a large amount of free amino acids for Streptococcus thermophilus S4 optimal growth,but also give fermented milk an excellent flavor,rich nutritional value and a certain acid tolerance.In addition,the genes and proteins relate to lactose metabolizes were highly expressed,especially ?-galactosidase and the gene of lac Z are significantly up-regulated,indicating that Streptococcus thermophilus S4 has a strong ability to metabolize galactose.It is speculated that this may be the main reason for the rapid rate of acid production.Besides,the pathways about nucleotide metabolism are highly induced in the pre-fermentation phase.These pathways can synthesize a large amount of nucleotide compounds which lacking in milk for the growth and utilization of Streptococcus thermophilus S4.In summary,this study can lay a theoretical foundation for better explanation and utilization of Streptococcus thermophilus S4,and provide a theoretical basis for optimizing the production process of fermented milk and improving the biomass of probiotics in fermented milk.