The intestinal microbiome in inflammatory bowel disease and spondyloarthritides

Extraintestinal manifestations (EIMs) of inflammatory bowel disease (IBD) can affect any organ system, ranging in presentation, frequency, and severity; the most common EIM is IBD-associated arthritis (IAA), an inflammatory arthritis affecting both axial and peripheral joints. IAA is a form of seronegative spondyloarthritis (SpA), which includes multiple arthritides, and those with IAA are also likelier to have other clinical features observed in those with SpA without IBD, such as increased rates of uveitis, psoriasis, and HLA-B27 positivity. These overlapping features are not limited to clinical disease features; the microbiome has been heavily implicated in the development of both IBD and SpA and is associated with non-SpA EIMs. In this review, we offer an overview of several bacterial taxa and associated metabolites that have been identified as differentially abundant in both IBD and SpA, both in stool and mucosal assessment, and how these taxa are associated with differences in intestinal permeability and immune profiles. We also describe how the overlapping dysbiotic microbiome and associated metabolome in IBD, EIMs of IBD, and SpA may differentially modulate the immune system, thus contributing to the shared underlying etiopathogenesis of these diseases. Further work is needed to characterize causal, mechanistic relationships beyond pure computationally identified associations, and apply these findings towards targeted therapeutic intervention and personalized medicine in IBD.