Recent advances in understanding the impact of intestinal microbiota on host behavior

The complex communities of enteric flora that colonize the human gastrointestinal (GI) tract play an important role in human health. Within the GI tract, microbiota have a mutually beneficial relationship with their host by modulating gut motility and secretion, as well as maintaining normal mucosal immune function, epithelial barrier integrity and nutrient absorption. Although the impact of enteric flora on regulating GI physiological activity is well known, how microbiota influence brain function and behavior is poorly understood. Accumulating data indicates that intestinal microbiota might communicate with the central nervous system (CNS) through immune, neuroendocrine and neural pathways, termed the gut-brain axis. Studies in germ-free animals and in animals exposed to pathogenic bacterial infections, probiotic bacteria or antibiotic drugs suggest a role for intestinal microbiota in the regulation of stress responses, anxiety, depression and cognition. Homeostasis of enteric flora composition improves host health, while disruption of this composition alters gut-brain functions and increases susceptibility to diseases, such as irritable bowel syndrome (IBS), inflammatory bowel disease (IBD), hepatic encephalopathy (HE), autism, Alzheimer's disease (AD), multiple sclerosis (MS) and chronic fatigue syndrome (CFS). In support of this proposition, increasing data from clinical studies revealed that enteric flora composition from patients of the above diseases are altered compared to healthy individuals, and microbiota dysbiosis can induce similar symptoms in animals. An increased understanding of the impact of microbiota on behavior could provide insights into the pathogenesis of gut-brain dysfunction (IBS, IBD and HE) and CNS disease (MS, autism and AD, etc.), and promote further exploration for the development of microbiota-based or microbiota-specific therapeutic strategies for CNS disease.