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 gutmicrobiota and psychiatric disorders. Articles in English with human participants up until February 13, 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 the 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, themost replicated findingswere higher abundance of Bifidobacterium and lower abundance of Roseburia and Faecalibacterium among patients with psychiatric disorders.

Conclusions: Gut bacteria that produce short-chain fatty acids, such as Roseburia and Faecalibacterium, could be less abundant in patients with psychiatric disorders, whereas commensal genera, for example, Bifidobacterium, might be more abundant compared with healthy controls. However, most included studies were hampered by methodological shortcomings including small sample size, unclear diagnostics, failure to address confounding factors, and inadequate bioinformatic processing, which might contribute to inconsistent results. Based on our findings, we provide recommendations to improve quality and comparability of future microbiota studies in psychiatry.