Recent studies of microbiota have indicated that gut microbiota might play an important role in the pathophysiology of irritable bowel syndrome (IBS). IBS is initiated by acute bacterial infection of the gut (post-infectious IBS). The composition of gut microbiota in IBS patients differs from that in healthy controls. Mimicking the features of IBS, psychosocial stress alters the composition of gut microbiota, increases mucosal permeability, and sensitizes visceral nociception. Not only IBS symptoms but also negative emotions, including depression, have been reported to be improved by interventions with oral administration of probiotics. Fecal concentrations of acetate and propionate are related to gastrointestinal symptoms, quality of life, and factor 3 of alexithymia. Continuing research into the role of gut microbiota in IBS is warranted.

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Depressive disorder is a chronic stress-induced condition, which has been suggested to have bidirectional interactions with the gut microbiota. Animal studies have suggested that probiotics mitigate stress responses, and ameliorate depressive-like behaviors and related abnormalities in the brain. Only a few studies have directly investigated the gut microbiota of patients with depressive disorder. We have reported that individuals with lower bacterial counts of Bifidobacterium and/or Lactobacillus are more common in patients with major depressive disorder than in healthy controls. In agreement with this, a clinical trial showing the beneficial effect of probiotics in the treatment of depressive disorder has come out. Children with autism spectrum disorder (ASD) often manifest gastrointestinal symptoms, and such symptoms are known to correlate with the severity of ASD, suggesting the involvement of gut microbiota in the pathophysiology of ASD. Several changes in the gut microbiota, such as altered bacterial counts of Clostridium and Sutterella, have also been reported, although results vary across studies. Gut microbiota-oriented therapies such as probiotics and fecal microbiota transfer therapy are ongoing for ASD children, which should lead to new horizons in the treatment of ASD.

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Chronic kidney disease (CKD) is a global health problem that carries a substantial risk for end- stage renal disease, cardiovascular disease, and death. In CKD, the accumulation of gut microbiota-derived uremic toxins [e.g. indoxyl sulfate, p-Cresyl sulfate, and trimethylamine N-oxide (TMAO)] accelerates the progression of CKD and hence, mortality. Therefore, reducing the accumulation of such uremic toxins through modification of gut microbiota is important for the protection against and amelioration of renal damage.

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