The microbiota-gut-brain axis in gastrointestinal disorders: stressed bugs, stressed brain or both?

Abstract

The gut-brain axis is the bidirectional communication between the gut and the brain, which occurs through multiple pathways that include hormonal, neural and immune mediators. The signals along this axis can originate in the gut, the brain or both, with the objective of maintaining normal gut function and appropriate behaviour. In recent years, the study of gut microbiota has become one of the most important areas in biomedical research. Attention has focused on the role of gut microbiota in determining normal gut physiology and immunity and, more recently, on its role as modulator of host behaviour ('microbiota-gut-brain axis'). We therefore review the literature on the role of gut microbiota in gut homeostasis and link it with mechanisms that could influence behaviour. We discuss the association of dysbiosis with disease, with particular focus on functional bowel disorders and their relationship to psychological stress. This is of particular interest because exposure to stressors has long been known to increase susceptibility to and severity of gastrointestinal diseases.

Figure 1. The microbiota–gut–brain axis comprises the bidirectional communication, through multiple pathways, between the gut and the brain

During stress, alterations at the level of the central nervous system can influence gut neuromotor and secretory function, immunity and microbiota composition. In turn, dysbiosis may contribute to perpetuate dysfunction and inflammation, further disrupting gut–brain communication. Some of these effects may be mediated by direct host–microbial interactions at the level of the intestinal epithelium, production of bacterial metabolites (cathecolamines, GABA, etc). The sequence of events can occur in a top-to-bottom or bottom-to-top fashion, but once initiated can perpetuate and exacerbate maladaptive responses that promote a state of disease. We acknowledge dreamdesign and cooldesign (http://FreeDigitalPhotos.net) for the image of the gut and brain, respectively.