Composition of Gut Microbiota and Its Relationship with Inflammatory Markers in Healthy Individuals and Patients with Ulcerative Colitis or Crohn’s Disease

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Background:The gut microbiota plays an important role in inflammatory bowel disease (IBD). However, its relationship with inflammatory markers remains unclear, especially in Asian populations. Methods:This prospective observational study examined the gut microbiota composition and its association with systemic inflammatory markers in Japanese patients with ulcerative colitis (UC) and Crohn’s disease (CD) compared to healthy controls. Bacterial diversity, short-chain fatty acid-producing bacteria, and key genera were analyzed via 16S rRNA gene sequencing. Results:Patients with CD exhibited reduced bacterial diversity. Clostridium cluster IV was decreased in CD and showed a negative correlation with fecal calprotectin levels in UC. Clostridium cluster XIVa was negatively correlated with fecal calprotectin levels in both UC and CD, and with C-reactive protein (CRP) levels in CD. Oscillibacter positively correlated with CRP levels in CD. These findings suggest that a reduction in butyrate-producing bacteria is associated with increased inflammation in patients with IBD. Conclusion:To our knowledge, this is the first study to demonstrate a significant association between specific bacterial taxa and inflammatory markers in Japanese patients with IBD. Clostridium clusters IV and XIVa may serve as biomarkers, although potential confounders such as age, sex, and concurrent medication use should be recognized as study limitations. J. Med. Invest. 73 : 92-100, February, 2026