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Background The microbiota most likely has an essential role in the pathogenesis of Crohn’s disease (CD). While faecal diversion after ileocecal resection (ICR) protects against CD recurrence, re-exposure triggers inflammation. After ICR, the majority of patients develop recurrence in the neoterminal ileum and the ileal microbiome is of particular interest. We have therefore assessed the mucosa-associated bacterial microbiome in the inflamed and non-inflamed ileum of patients with CD. Methods Patients with an established diagnosis of CD undergoing ileocolonoscopy and healthy controls (HC) referred for colonoscopy due to rectal bleeding or screening for disease, but without any detected gastrointestinal pathology were invited to participate. Exclusion criteria included use of antibiotic treatment for the past 2 months. Mucosal pinch biopsies were sampled 5 cm and 15 cm orally of the ileocecal valve or ileocolic anastomosis for comparisons within the same patients. The biopsies were analysed by 16S rRNA sequencing, α- and β-diversity was assessed by Shannon entropy and Bray-Curtis dissimilarity index respectively. Histologic inflammation was graded. Results Fifty-one CD patients, where of 32 with previous ICR, and 40 HC were included in the study. Of the 51 CD patients, 20 had terminal ileitis, with endoscopically inflamed mucosa at 5 cm and normal appearing mucosa at 15 cm and no history of upper GI disease involvement. Seven CD patients had ileal stenosis. CD patients (n = 51) had lower α-diversity and separated clearly from HC on β-diversity plots (Figure 1). Twenty-three bacterial taxa were differentially represented in CD patients and HC, among these Tyzzerella 4 was found to be profoundly overrepresented in CD (p = 4.1 × 10–68). When comparing the microbiome in the inflamed ileal mucosa with the proximal non-inflamed mucosa within CD patients (n = 20) neither α- or β-diversity differed (Figure 2). Furthermore, no bacterial taxa were differentially represented in the inflamed vs. proximally non-inflamed mucosa. CD patients operated with ICR had lower α-diversity (p = 0.021), but β-diversity did not differ from CD patients without ICR. CD patients with stenosis had lower abundances of Bacteroides, Sutterella and Akkermansia species. Conclusion 23 taxa were differentially expressed in CD compared with HC. The ileal mucosa-associated microbiome is similar assessed by both α- and β-diversity in the inflamed mucosa and the proximal non-inflamed mucosa within the same patients. Our results support the concept of CD specific microbiota alterations and demonstrate that neither ileal sub-location nor endoscopic inflammation itself influence the mucosa-associated microbiome.

P833 Bacterial mucosa-associated microbiome in inflamed and proximal non-inflamed ileum of patients with Crohn’s disease