Lactobacillus salivarius ZDY159a fermentation broth,fermented supernatant fluid,live L.salivarius ZDY159a suspended into PBS were studied to detect the antibacterial activity in vitro based on co-culture experiment,using Escherichia coli O157:H7 NCTC 12900 and Staphylococcus aureus CMCC 26003 as indicator organisms.The type and concentration of main organic acids from fermented supernatant fluid of L.salivarius ZDY159a were analysed by HPLC.Lactic acid and acetic acid,same concentration as the fermented supernatant fluid respectively,were studied to detect the antibacterial activity in vitro by Oxford cup method,using E.coli O157:H7 NCTC 12900 and S.aureus CMCC 26003 as indicator organisms.The antibacterial activity change of fermented supernatant fluid after pH adjustment,protease digestion and catalase treatment respectively was studied in vitro by Oxford cup method,using E.coli O157:H7 NCTC 12900 and S.aureus CMCC 26003 as indicator organisms.It was found that fermentation broth and fermented supernatant fluid have the strong inhibition to indicator organisms.Moreover,the antibacterial activity of fermentation broth was more stronger than that of fermented supernatant fluid.The main organic acids from fermented supernatant fluid were 0.200 mol/L lactic acid and 0.162 mol/L acetic acid.The concentration of lactic acid and acetic acid have the strong inhibition to indicator organisms.Moreover,the anti-bacterial activity of acetic acid was more stronger than that of lactic acid.The antibacterial activity of fermented supernatant fluid at low pH was enhanced.The antibacterial activity of fermented supernatant fluid after protease digestion and catalase treatment respectively was decreased.In conclusion,it was speculated that the antibacterial activity of L.salivarius ZDY159a was attributed to organic acids,bacteriocin or proteins and hydrogen peroxide from fermented supernatant fluid,and Low pH is a key condition for antibacterial activity of L.salivarius ZDY159a.