To understand the role of mucosa-associated microbiota in the pathogenicity of ulcerative colitis (UC), paired biopsies were obtained during colonoscopy from the ulcerated and nonulcerated gut mucosa of 24 patients with UC. Denaturing gradient gel electrophoresis analysis was employed to profile the composition of the dominant bacteria (16S rRNA gene V3 region) and three important groups: lactobacilli, the Clostridium leptum subgroup, and Bacteroides spp. The Pearson coefficient was used to estimate similarities between the bacterial communities of the paired biopsies for each patient. The average similarity values of bacterial composition between the paired samples were 94.8 ؎ 3.8% for dominant bacteria, 59.9 ؎ 26.1% for lactobacilli, 79.2 ؎ 22.6% for the Clostridium leptum subgroup, and 88.7 ؎ 16.4% for Bacteroides spp. The data revealed that lactobacilli and the Clostridium leptum subgroup were significantly different between the ulcerated and the nonulcerated regions. It also was noted that for lactobacilli, the composition varied significantly between biopsy sites irrespective of the location of UC in the gut but that the composition of the Clostridium leptum subgroup showed significant differences between paired samples from UC in the rectum and not in the left colon. Localized dysbiosis of the mucosa-associated intestinal microflora, especially for lactobacilli and the Clostridium leptum subgroup, may be closely related to UC.

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