Gut bacteria play a key role in initiating and maintaining the inflammatory process in the gut tissues of inflammatory bowel disease (IBD) patients, by supplying antigens or other stimulatory factors that trigger immune cell activation. Changes in the composition of the intestinal microbiota in IBD patients compared to that in healthy controls and a reduced diversity of intestinal microbial species are linked to the pathogenesis of IBD. Adherent invasive (AIEC) has been linked to Crohn's disease (CD) patients, while diffusely adherent (DAEC) has been associated with ulcerative colitis (UC). Bacteriological analysis of intestinal biopsy specimens and fecal samples from IBD patients shows an increased number of strains belonging to the B2 phylogenetic group, which are typically known as extraintestinal pathogenic (ExPEC). Results from studies of both cell cultures and animal models reveal pathogenic features of these pathobionts, which may link them to IBD pathogenesis. This suggests that IBD-associated strains play a facilitative role during IBD flares. In this review, we explain IBD-associated and its role in IBD pathogenesis.