An approach to finding specific forms of dysbiosis that associate with different disorders.

BackgroundMany disorders display dysbiosis of the enteric microbiome, compared with healthy controls. Different disorders share a pattern of dysbiosis that may reflect reverse causation, due to non-specific effects of illness-in-general. Combining a range of disorders into an aggregate non-healthy active control (ANHAC) group should highlight such non-specific dysbiosis. Differential dysbiosis between the ANHAC group and specific disorders may then reflect effects of treatment or bowel dysfunction, or may potentially be causal. Here, we illustrate this logic by testing if individual genera can differentiate an ANHAC group from two specific diagnostic groups. MethodsWe constructed an ANAHC group (n=17) that had 14 different disorders. We then used random forest analyses to test differential dysbiosis between the ANHAC group and two other disorders that have no known pathology, but: (i) symptoms of illness (Myalgic Encephalomyelitis / Chronic Fatigue Syndrome - ME/CFS - n = 38); or (ii) both illness and bowel dysfunction (ME/CFS comorbid with Irritable Bowel Syndrome - IBS - n=27). ResultsMany genera differentiated the ANHAC group from co-morbid IBS. However, only two genera - Roseburia and Dialister - discriminated the ANHAC group from ME/CFS. ConclusionsDifferent disorders can associate with specific forms of dysbiosis, over-and-above non-specific effects of illness-in-general. Bowel dysfunction may contribute to dysbiosis in IBS via reverse causation. However, ME/CFS has symptoms of illness-in-general, but lacks known pathology or definitive treatment that could cause dysbiosis. Therefore, the specific dysbiosis in ME/CFS may be causal. [230 words] Contribution to the fieldMany disorders associate with enteric dysbiosis. The pattern of dysbiosis is largely consistent between unrelated disorders, which suggests that it mainly reflects non-specific secondary effects of illness-in-general (e.g. due to changes in activity levels, or diet). However, faecal microbiome transplantation (FMT) can be therapeutic in some disorders. This implies that unique features of dysbiosis may cause those specific disorders. Here, we propose a way to assess causal effects of dysbiosis, by testing if individual genera can discriminate individual disorders from an aggregate non-healthy active control (ANHAC) group. Dysbiosis in the ANHAC group can control for non-specific effects of illness-in-general on the microbiome and so highlight potentially-causal forms of dysbiosis in specific disorders. This approach may provide insight into pathogenetic mechanisms of individual disorders and help to design specific forms of FMT to counteract them.