Biodiversity And Antioxidant Properties Of Lactic Acid Bacteria From Spontaneously Fermented Yak Milk In The Qinghai-Tibet Plateau

Under the extreme environmental conditions of strong ultraviolet and low oxygen of the Qinghai-Tibet Plateau,the spontaneously fermented yak yoghurt with a history of thousands of years is rich in lactic acid bacteria?LAB?germplasm resources.The yak yogurt samples were collected from the of five ecological geographic areas of the Qinghai-Tibet Plateau in 2012 and 2013,and totally 241 samples of yak yogurt were collected for isolation of lactic acid bacteria.All the isolated lactic acid bacteria?1912strains?were purified and identified by 16S rRNA gene sequence and the germplasm library of lactic acid bacteria from fermented yak yoghurt on the Qinghai-Tibet Plateau was established.Thereafter,the diversity of lactic acid bacteria and their antioxidant properties were studied.A total of 1912 strains of lactic acid bacteria were identified by 16S rRNA gene sequencing,belonging to 7 genera and 28 species or subspeceis.Among them,there were 1017 strains of Lactobacillus,accounting for 53.19%of the isolated strains.There were 617 strains of Pediococcus,accounting for 32.27%.Enterococcus and Leuconostoc were 244 and 27 strains respectively,accounting for 12.76%and 1.41%.There were 3 strains of Streptococcus,which only accounted for 0.16%of the isolated strains.In 27 species,Lactobacillus genus was mainly comprised by L.plantarum,L.reuteri and L.delbrueckii.There were 606 strains of Lactococcus lactis in traditional fermented yak yoghurt in the Qinghai-Tibet Plateau,accounting for 31.69%of all identified strains.When the initial H₂O₂ concentration in the medium was 1.9mmol/L,there were21 strains with OD values above 1.00 after 8h of growth.All strains had DPPH clearance of above 83%.Based on the DPPH clearance,OH radical removal force,O₂^-the clearance of free radicals,reducing power and strain of total antioxidant capacity and T-SOD activity of lactic acid bacteria fermentation supernatant,bacteria cells and cell free extracts on,10 strains were finally screened out with high antioxidation characteristics and these strains were As21,Bs44,Fm15,As39,Bs48,D57,D61,Bs29,As13,D28.In the in vitro probiotics experiments,the survival rate of 10 strains after growing in the MRS medium with pH of 3 for 3 hours was more than 94%.The strain with the highest hydrophobicity was D57,which was 51.12%.The hysteresis period of strain As21 was only 2.85 h.The 10 strains of lactic acid bacteria were digested in the artificial gastric juice at pH 3.0 for 3h and then transferred to the artificial intestinal juice for 24 h.The survival rate of As21 was 64.28±0.09%,and that of Bs44 was57.49±0.08%.Finally,two strains of Lactococcus lactis As21 and Lactobacillus plantarum Bs48 were screened with high antioxidant activity and with excellent in vitro probiotic profiles.