Does IBD location within the intestines affect the gut microbiome? Original paper

Inflammatory bowel disease (IBD) is associated with structural and functional changes in the gut microbiome. However, whether microbiome diversity, taxonomy, or functionality differs based on disease location within the intestinal tract is unclear. Therefore, this cohort study assessed gut microbiome diversity and composition differences between IBD subtypes and investigated whether disease location was associated with differences in gut microbiome diversity or composition.

The investigators collected 175 stool samples from participants with Crohn's disease (CD), ulcerative colitis (UC), microscopic colitis (MC; a rare form of IBD),^[1] and healthy control participants to assess gut microbiome diversity and composition and used subjective scoring systems to categorized disease severity as inactive, mild, medium, or severe.

Participants with CD and UC were further stratified by disease location (there weren't enough patients with MC to stratify these patients by disease location):

• Small bowel Crohn's disease (CD-SB)
• Terminal ileitis Crohn's disease (CD-TI)
• Colonic Crohn's disease (CD-CC)
• Crohn's disease of multiple sites (CD-multi)
• Left-side ulcerative colitis (UC-LS)
• Pancolitis (inflammation of the entire colon, UC-PC)
• Ulcerative proctitis (inflammation of the lining of the rectum, UC-PR)

The participants with CD-CC and CD-SB had lower absolute species diversity (ASV) and phylogenic diversity than those with CD-TI. The participants with CD-TI had similar diversity measures as healthy control participants. Similar differences were observed between groups for taxonomic composition, as assessed using the Bray-Curtis dissimilarity index.

None of the UC locations differed in microbiome diversity or taxonomic composition as compared with one another or with healthy controls.

There was no relationship between disease severity and ASV or phylogenic diversity. However, the authors noted that relatively few participants had moderate or severe disease.

Participants with CD-TI had a greater abundance of Faecalibacterium relative to other CD locations, participants with CD-CC had greater abundance in Streptococcus and Burkholderia relative to other disease locations and healthy controls, and participants with CD-SB had a greater abundance of Escherichia and Acinetobacter relative to other disease locations and healthy controls.

Blautia, Eubacterium, and Collinsella were more abundant in participants with UC relative to healthy controls, with no differences in taxa across UC disease locations. In participants with MC, Alistipes was more abundant than in healthy controls.

Participants with UC had a reduction in the pathways responsible for butyrate production relative to healthy controls. The participants with CD-TI were enriched in pathways for pyruvate to butyrate fermentation, those with CD-SB were enriched in pathways for succinate fermentation to butyrate, and those with CD-CC were enriched for pathways for beta-lactam resistance and menaquinone (vitamin K2 production). There were no functional differences between different UC locations.