Abstract: Gut microbiota changes were investigated in the present study after infection with Escherichia coli O157∶H7 and treated with Lactobacillus acidophilus KLDS AD1. Appearance characteristics were employed to identify the preventive roles of L.acidophilus KLDS AD1 in the mice infected with E.coli O157 ∶H7. Mice were randomly allocated to nonchallenged control (Ⅰ) , E.coli-PBS treatment (Ⅱ) and E.coli- L.acidophilus treatment ( Ⅲ) groups. After oral administration of E. coli O157 ∶ H7 for 7 days, the infected mice were fed with L.acidophilus KLDS AD1 for another 7 days, and the numbers of bacteria in the faeces of mice were counted. Results indicated that L.acidophilus KLDS AD1 could result in the weight gain significantly of E.coli O157∶H7 infected mice (p<0.01) . Faecal Lactobacillus, Bifidobacterium counts were significantly decreased (p<0.05) , while Enterococcus, Enterobacteria and E.coli counts were obviously increased (p<0.05) , in groupsⅡand Ⅲ after E.coli O157∶H7 challenge compared with groupⅠ. After oral administration of L.acidophilus KLDS AD1, the number of Enterococcus, Enterobacteria and E.coli were significantly lower (p<0.05) , while Lactobacillus, Bifidobacterium counts were obviously higher (p<0.01) in groups Ⅲ than groupsⅡ. These results demonstrated that L.acidophilus KLDS AD1 was effective in regulating the intestinal microbiota and ameliorating diarrhea in E.coli O157∶H7 infected mice.