Gut Microbiota in Psychiatric Disorders: A Systematic Review.

Objective This systematic review sought to comprehensively summarize gut microbiota research in psychiatric disorders following PRISMA guidelines. Methods Literature searches were performed on databases using keywords involving gut microbiota and psychiatric disorders. Articles in English with human participants up until 13th of February 2020 were reviewed. Risk of bias was assessed using a modified Newcastle-Ottawa scale for microbiota studies. Results Sixty-nine of 4231 identified studies met inclusion criteria for extraction. In most studies, gut microbiota composition differed between individuals with psychiatric disorders and healthy controls; however, limited consistency was observed in the taxonomic profiles. At the genus level, the most replicated findings were higher abundance of Bifidobacterium, and lower abundance of Roseburia and Faecalibacterium among patients with psychiatric disorders. Conclusion Gut bacteria that produce short-chain fatty acids, such as Roseburia and Faecalibacterium, could be less abundant in patients with psychiatric disorders, while commensal genera, for example Bifidobacterium, might be more abundant compared to healthy controls. However, most included studies were hampered by methodological shortcomings including small sample size, unclear diagnostics, failure to address confounding factors, and inadequate bio-informatic processing, which might contribute to inconsistent results. Based on our findings, we forward recommendations to improve quality and comparability of future microbiota studies in psychiatry.