Background: The role of the intestinal microbiota in the pathogenesis of inflammatory bowel disease combined with axial spondyloarthritis (axSpA) is gaining widespread interest. Aims: This study was conducted to investigate the clinical and fecal microbiota characteristics of patients with both ulcerative colitis (UC) and axSpA. Methods: Clinical data were collected from patients with UC. Patients were divided into the axSpA and non-axSpA groups according to human leukocyte antigen-B27 serology and sacroiliac joint imaging results. We obtained fecal specimens from 14 axSpA and 26 non-axSpA patients. All samples underwent 16S ribosomal DNA sequencing. Results: Seventy-three patients with UC were included in this study, and the axSpA incidence was 19.2%. This incidence was significantly higher in patients with C-reactive protein > 10 mg/L. Firmicutes and Faecalibacterium abundances were decreased, and Proteobacteria and Escherichia_Shigella abundances were increased in the axSpA group compared with those of the non-axSpA group. Indicator analysis showed that Escherichia_Shigella was more likely to be an indicator species of axSpA. Additionally, many biosynthetic and metabolic pathways, including glutathione metabolism, fatty acid degradation, geraniol degradation, and biosynthesis of siderophore group nonribosomal peptides, were upregulated in the axSpA group. Conclusion: Patients with UC have a high axSpA incidence, which may be related to the relative abundances of Escherichia_Shigella in these patients. The abundances of various biosynthetic and metabolic pathways of the fecal flora were upregulated in patients with axSpA.

Purpose: Ulcerative colitis (UC) requires new non-invasive serum biomarkers for assistance in monitoring due to the low rate of endoscopic follow-up. Previous research indicated reduced levels of serum indirect bilirubin (sIBIL), serum total bilirubin (sTBIL), and serum total bile acids (sTBAs) in UC patients. This study aims to assess their monitoring potential in UC during biologic therapy. Methods: We conducted a retrospective single-center study including 138 UC patients and 150 controls with normal colonoscopy results. The receiver operating characteristic (ROC) curve was used to assess diagnostic value of sIBIL, sTBIL, and sTBAs. Spearman correlation analysis was performed to assess the association between these biomarkers and the severity of both endoscopic findings and clinical symptoms in UC patients. Additionally, changes in serum biomarkers were analyzed in 72 UC patients during biologic therapy, with stratified analyses based on endoscopic remission status. Results: Patients with UC exhibited lower concentrations of sIBIL, sTBIL, and sTBAs compared to the controls (P < 0.05), and all these biomarkers demonstrated moderate diagnostic value in identifying UC from normal controls (P < 0.05). sIBIL concentration negatively correlated with disease severity and showed a progressive increase during biologic therapy, particularly in patients achieving endoscopic remission at week 52 (P < 0.05). The sIBIL concentration in the remission group was significantly higher than that in the non-remission group after week 26 (P < 0.05). For sTBAs, concentration initially increased and then decreased, with a turning point at week 14 in the remission group (P < 0.05) and at week 26 in the non-remission group (P > 0.05). No significant differences in sTBAs concentrations were found between remission and non-remission groups at any time (P > 0.05). Conclusion: sIBIL may be used as a valuable serum biomarker for the clinical diagnosis and the monitoring of response to biologics. Additionally, the change trend of sTBAs may provide reference value for monitoring UC biologic therapy. However, further studies are needed to analyze the changes in its internal composition.

Background: Patients with ulcerative colitis (UC) frequently exhibit gut enrichment of Enterobacteriaceae and reduced serum bilirubin levels. However, whether bilirubin statistically accounts for the association between Enterobacteriaceae abundance and UC disease activity remains unclear. Methods: This study enrolled 81 UC patients. The relative abundances of Enterobacterales lineage ( Gammaproteobacteria , Enterobacterales , Enterobacteriaceae , and Escherichia-Shigella ) were determined by 16S rRNA sequencing, and disease activity was assessed using the Mayo endoscopic score (MES). Total bilirubin (TBIL) and indirect bilirubin (IBIL) were examined as potential associative variables. Associational variable analyses were performed using bootstrap methods with 5,000 resamples, and sensitivity analyses were conducted after adjusting for covariates. Results: The abundances of Enterobacterales lineage were positively correlated with MES ( p = 0.34–0.40) and negatively correlated with TBIL and IBIL ( p = −0.31 to −0.48). Associational variable analyses showed that bilirubin significantly and partially accounted for these positive associations. In covariate-adjusted models, the associational proportions ranged from 40.8% to 42.3% for TBIL and from 37.2% to 40.0% for IBIL. Conclusion: Bilirubin statistically accounts for approximately 37.2%–42.3% of the association between Enterobacterales lineage abundance and UC disease activity. Owing to the cross-sectional design, these findings reflect statistical covariation rather than causal effects, and prospective studies are needed to establish temporal direction.