The microbiome reflects diagnosis and predicts disease severity in paediatric onset inflammatory bowel disease

<b>Objectives:</b> A microbiotic profile characterized by decreased abundance and richness has been described in inflammatory bowel disease (IBD). Recently, sequencing the microbiome to the species level has become possible, which can improve our understanding of the gut to host interaction in IBD. We aimed to describe the microbiotic profile in paediatric IBD and compare it to disease phenotype and disease course. <b>Methods:</b> Faecal samples were collected from a cross-sectional cohort. The microbiome analysis was performed using 16S and 18S rRNA sequencing with the miSeq instrument. Inflammatory activity was assessed by faecal calprotectin. Data regarding medical treatment and surgery in the year after faecal sampling were collected from patient charts. <b>Results:</b> One hundred and forty-three (143) paediatric IBD patients and 34 healthy controls (HC) were included. We found a reduced richness in IBD patients compared to HCs (controls vs. ulcerative colitis (UC), <i>p</i> p = .04)). Moreover, a high degree of intestinal inflammation and extensive disease extent was associated with reduced richness in UC (<i>p</i> = .02 and <i>p</i> = .04, respectively). Nine species were significantly associated with a healthy microbiome and three species were associated with IBD. Lastly, we found that the composition of the microbiome could distinguish between CD, UC and HCs. <b>Conclusions:</b> In this study, we found that the microbiome could discriminate between IBD phenotypes and predict which patients were at risk of surgery. In the future, this could be included as part of the diagnostic work-up in IBD patients.