Objective:To investigate the effects of adherent-invasive Escherichia coli ( E. coli) LF82 on the structure and function of intestinal barrier in mice with ulcerative colitis (UC). Methods:Twenty-four specific pathogen free (SPF) C57BL/6 mice were divided into UC with E. coli LF82 group, UC group and healthy control group with eight mice in each group. The UC mice model was induced by dextran sulfate sodium (DSS). One week before modeling, the mice of UC with E. coli LF82 group were intragastric administrated with 1×10 9 colony-forming unit (CFU) E. coli LF82 to colonize the bacteria strain. The effects of E. coli LF82 on colitis of mice with UC were evaluated by disease activity index (DAI), gross morphological injury score, colonic mucosal injury index (CMDI), myeoloperoxidase (MPO) activity and pathological features. The ultrastructure and the changes of cytoskeleton F-actin of mice colonic tissues were detected by transmission electron microscope (TEM) and direct immunofluorescence. The ability of colonic mucin production and degree of fibrosis were estimated by periodic acid Schiff reaction (PAS) stain and sirius red stain. T test, least significant difference, repeated measurement analysis of variance and one-way analysis of variance were used for statistical analysis. Results:On the fourth, fifth, sixth and seventh day after the modeling, the DAI scores of UC with E. coli LF82 group were all higher than those of UC group ((2.53±0.38) points vs. (2.01±0.53) points, (3.02±0.62) points vs. (2.67±0.24) points, (3.13±0.61) points vs. (2.20±0.24) points, (3.27±0.28) points vs. (2.20±0.69) points, respectively), and the differences were statistically significant ( t=3.37, 2.25, 9.56 and 10.24, all P<0.05). The gross morphological injury score of mice colon of UC with E. coli LF82 group was higher than that of UC group ((6.17±1.94) points vs. (2.83±0.98) points), and the difference was statistically significant ( t=-3.75, P<0.05). The CMDI and MPO activity of UC with E. coli LF82 group were both higher than those of UC group ((16.80±2.79) points vs. (11.80±3.11) points, (729.3±77.5) U/mg vs. (594.4±31.9) U/mg), and the differences were statistically significant ( t=-2.83; mean difference=134.82, 95% confidence interval ( CI) 72.12 to 197.51; both P<0.05). The results of TEM showed that the E. coli LF82 could invade the submucosa of colon and caused further injury of colonic tissues in mice. The distribution of cytoskeleton F-actin of mice colonic tissues changed. The results of PAS staining showed that the percentages of PAS positive cells of UC with E. coli LF82 group and UC group were both lower than that of healthy control group ((32.40±8.02)% and (41.90±8.99)% vs. (57.70±11.52)%), and the difference was statistically significant ( F=17.63, P<0.01). The percentage of PAS positive cells of UC with E. coli LF82 group was lower than that of UC group, and the difference was statistically significant (mean difference=-9.50, 95% CI -18.33 to -0.67, P<0.05). The results of sirius red staining showed that the villous epithelium of colon mucosa of UC with E. coli LF82 group was partially injured and collagen fibers hyperplasia was serious. The area ratios of collagen fiber of UC with E. coli LF82 group and UC group were both higher than that of healthy control group ((51.83±5.78)% and (37.11±5.59)% vs. (15.41±2.25) %), and the difference was statistically significant ( F=86.72, P<0.01). The area ratio of collagen fiber of UC with E. coli LF82 group was higher than that of UC group, the difference was statistically significant (mean difference=14.83, 95% CI 8.91 to 20.76, P<0.05). Conclusions:E. coli LF82 can aggravate DSS-induced colitis in UC mice, leading to changes in colon ultrastructure and cytoskeleton, it can also reduce the ability of mucus secretion of colon of mice and increase the degree of colonic tissues fibrosis.