A Microbial Relationship Between Irritable Bowel Syndrome and Depressive Symptoms.

Irritable bowel syndrome (IBS) is associated with depressive symptoms, but this relationship is poorly understood. Emerging research suggests that gut microbes are associated with symptoms in persons with IBS. The purpose of this integrative review is to describe the state of the science of the microbial relationship between IBS and depressive symptoms. PubMed, CINAHL, PsychINFO, and Web of Science were searched using "irritable bowel syndrome," "microbiome," "depression," and related terms. Included articles were published in peer reviewed journals in English from 2009 to 2018. Studies on inflammatory bowel conditions, extra-intestinal microbiomes, or animal models were excluded. Fourteen quantitative studies met inclusion criteria, were critically appraised, and were analyzed using the Whittemore and Knafl method. Analysis revealed a consistently lower microbial biodiversity and lower proportions of Bifidobacterium and Lactobacillus in persons with IBS and co-occurring depressive symptoms. Inclusion of participants with moderate or greater depressive symptoms scores distinguished the studies which reported microbe differences in depressive symptoms. The results of this integrative review underscore the need for studies with larger samples and inclusion of a larger range of depressive symptoms guided by an overarching conceptual framework, such as the biopsychosocial ecology framework. This effort needs to be combined with longitudinal designs in order to identify related microbial markers.