Abstract: A total of 113 strains of lactic acid bacteria (LAB) were isolated from yak milk dreg in Tibet. Out of these, four dominant LAB stains were screened and compared for their exopolysaccharide (EPS)-producing ability, acid, bile and osmotic pressure tolerance, and antimicrobial activity. Results demonstrated that the EPS-producing abilities were different for the 4 selected strains, with strain GY-L003 producing the highest yield of EPS (379.94 mg/L). The LAB strains exhibited different levels of tolerance to three environmental factors and strains GY-L003 and GY-L004 could survive at extremely acidic pH of 1.5. At a bile concentration of higher than 0.4%, GY-L003 and GY-L004 could grow while GY-L055 and GY-L112 were inhibited. In terms of osmotic tolerance, GY-L003 and GY-L004 were superior, being able to grow when NaCl concentration was up to 9%. All four LAB strains had strong antimicrobial activity, among which GY-L004 had the strongest inhibitory effect against Staphylococcus aureus, with an inhibition zone diameter of (15.94 ± 0.31) mm, GY-L003 had the strongest inhibitory effect against both E. coli and Salmonella, with an inhibition zone circle of (18.21 ± 0.65) and (16.27 ± 0.50) mm, respectively, and GY-L112 exhibited the strongest inhibitory effect against Listeria monocytogenes with an inhibition zone diameter of (15.24 ± 0.35) mm. Taken collectively, our results suggested that both GY-L003 and GY-L004 had high EPS-producing ability, good tolerance to acid, bile and osmotic pressure, and strong antimicrobial activity. Accordingly, these strains have a good application potential and can provide a reference basis for application in the production of fermented dairy products.

Key words: lactic acid bacteria, exopolysaccharide, tolerance, antimicrobial activity