Bipolar Disorder and the Gut Microbiome: a Systematic Review.

Objectives The microbiome is a rapidly advancing biomedical frontier with relevance for psychiatric illness. The gut microbiota interact with the central nervous system bidirectionally through the gut-brain axis and generate substances that may influence host metabolism, including short-chain fatty acids such as butyrate. Understanding gut microbiota in bipolar disorder (BD) may suggest new disease markers and treatment approaches. Methods A PubMed search was performed on January 7, 2020 using terms "bipolar AND (microbiome OR microbiota)", for articles in English in which the study population included a distinct BD group and the gut microbiota/microbiome was assessed. Results Thirteen articles met the inclusion criteria. In four out of five studies that reported on group comparisons with respect to diversity, lower α-diversity was observed in BD relative to healthy controls (HC). The most convergent taxonomic finding was that in four studies, one particular clade distinguished gut microbiota between BD and HC: family Ruminococcaceae, genus Faecalibacterium, and species Faecalibacterium prausnitzii. Members of this clade, known for butyrate production, were reduced in BD relative to HC in three studies but elevated in a fourth. Additionally, genera Bacteroides or Bacteroides-Prevotella group species were elevated in BD in two studies but lower in a third. Conclusions Despite few studies and modest sample sizes, salient findings suggest that low α-diversity and dysbiosis with respect to abundance of Faecalibacterium and Bacteroides may characterize BD in both a trait and state-dependent fashion. Decreased richness and butyrate production also foster inflammation, which may be a hitherto unrecognized part of the pathophysiology underlying BD.