Isolation,Identification And Functional Characteristics Of Streptococcus Thermophilus DMST-H2

Streptococcus thermophilus,a common fermentation strain with long application history,is widely used in the production of fermented dairy products.Nowadays,China's fermented milk market is expanding year by year,functional fermented milk products are becoming more diversified,and sales are increasing.Therefore,the demand for good starter is also increasing.However,the domestic starter market has been monopolized by multinational companies,because of the late started research and development of starter technology in China.At present,domestic research on Streptococcus thermophilus is mostly focused on strain screening and fermentation process optimization,and there is a lack of systematic research on genome and functional properties.This research aims to develop a strain of Streptococcus thermophilus with independent intellectual property rights,native origin,good fermentation and functional properties.And evaluate its genome,fermentation and functional properties,aiming to provide a theoretical basis for its future applications.We hope this research can further enhance the safety and core competitiveness of our country's dairy industry.Firstly,the strain DMST-H2 was isolated from the traditional Chinese fermented milk,the Inner Mongolia home-made fermented milk,and was identified as Streptococcus thermophilus.The whole genome sequencing analysis performed that it contained a circular chromosome of 1.88 Mb,with an average G+C content of 38.98%,and a total of 2016 coding genes,most of which were annotated as metabolism-related genes.The comparative genome analysis showed that the genome structure of Streptococcus thermophilus DMST-H2 was similar to the reference strains,but it had 40 unique genes.Secondly,this research systematically evaluated the safety,fermentation performance,gastrointestinal tolerance and functional features of Streptococcus thermophilus DMST-H2 based on in vitro test and functional gene annotation results.The results showed that Streptococcus thermophilus DMST-H2 had no antibiotic resistance,hemolysis,biogenic amine production ability and related genes,no D-lactic acid production,no oral toxicity in mice.It was safe and non-toxic.After 12 h of fermentation in skim milk medium,the p H dropped to 4.43±0.01.The kinds of producted acids included formic acid,acetic acid,propionic acid,isobutyric acid,isovaleric acid,L-lactic acid,etc.At the genetic level,Streptococcus thermophilus DMST-H2 encoded a variety of carbohydrate metabolism genes such as fructose,mannose,galactose,and other metabolic pathways such as glycolysis.The gene annotation results showed that Streptococcus thermophilus DMST-H2 contained genes related to gastrointestinal adaptability.The in vitro simulation test showed that the survival rate of DMST-H2 in the artificial gastrointestinal tract was still 82.04±2.00% after 5 hours.In addition,the self-cohesion of DMST-H2 reached 82.06±2.50% at 4 h,the DPPH clearance rate of the fermentation supernatant was 94.06±1.20%,and the co-precipitation rate of Escherichia coli ATCC 25922 and Staphylococcus aureus RN4220 was 22.21±0.66% and61.28±2.01%,respectively.Finally,in view of the frequent clinical symptoms of antibiotic-related diarrhea,this research explored the effect of Streptococcus thermophilus DMST-H2 on antibiotic-related diarrhea in mice.After lincomycin hydrochloride gavaging,mice showed loose stools,increased fecal water content,decreased content of Bifidobacteria and Lactobacilli than Enterobacteriaceae and Enterococci in feces.The antibiotic-related diarrhea mouse model was successfully established.After gavaging of Streptococcus thermophilus DMST-H2,symptoms such as cecal enlargement and intestinal barrier damage were effectively alleviated,and the levels of endotoxin,D-lactic acid and diamine oxidase were significantly reduced.In addition,the content of pro-inflammatory factor TNF-? decreased,the content of IFN-? and anti-inflammatory factor IL-10 increased.The results of intestinal flora using 16 s r DNA amplicon sequencing showed that the structure of the intestinal flora was restored after gavaging of Streptococcus thermophilus DMST-H2,which performed as the content of Firmicutes increased and Proteobacteria decreased.Compared with the natural recovery group,gavaging of Streptococcus thermophilus DMST-H2 better promoted the recovery of antibiotic-related diarrhea.In summary,Streptococcus thermophilus DMST-H2 was isolated from traditional Chinese fermented milk.And its safety,fermentation characteristics,gastrointestinal tolerance and functional properties were verified both at the genetic level and in vitro experiments.Streptococcus thermophilus DMST-H2 had a positive effect in restoring the intestinal flora,organ damage,and immune factor levels in antibiotic-related diarrhea mice.The above showed that Streptococcus thermophilus DMST-H2 was a native Chinese strain with with good potential function and potential application value.