Although the gut microbiome of patients with ulcerative colitis (UC) has been characterized, no study has characterized the gut microbiome in acute severe colitis (ASC). We compared the gut microbiome of patients with UC, ASC and healthy controls (HC).
Patients with mild to moderate UC (n=24), ASC (n=19 with 21 episodes) and healthy controls (n=50) were recruited prospectively. A 16SrDNA amplicon approach was used to explore gut microbial diversity and taxonomic repertoires. Ulcerative colitis was diagnosed using ECCO guidelines and ASC was diagnosed using Truelove and Witts’ criteria.
The normalized alpha diversity was significantly lower in ASC than mild-moderately active UC (p<0.05), or HC (p<0.001). The gut microbiome in ASC was highly unstable, as characterized by high intra-cohort variation (analyzed using J-divergence measure) which was significantly greater than in UC or HC. On principal coordinate analysis, the microbiome of HC and UC were similar, with the ASC cohort being distinct from both. Comparison of ranked abundances identified four distinct clusters of genera (G1, G2, G3, G4), with specific trends in their abundance across 3 groups: G1/G2A clusters had the least, whereas G3 had the highest abundance in the ASC cohort.
Gut microbial diversity is lower in ASC than mild-moderate UC or healthy controls. Gut microbiome composition is increasingly unstable in ASC, with a distinct abundance of specific genera varying between healthy controls and ASC. Mild-moderate UC lies within the spectrum.

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