The microbial organ in the gut as a driver of homeostasis and disease.

Based on the ability of bacteria to both recognize and synthesize neuroendocrine hormones, it is hypothesized that microbes within the intestinal tract comprise a community that interfaces with the mammalian nervous system that innervates the gastrointestinal tract to form a microbial organ. Given the evolutionary context in which the central nervous system is an outgrowth of the more primitive enteric nervous system and the time in which microbes have colonized the mammalian intestinal tract, it is further hypothesized that this microbial organ enters into a symbiotic relationship with its mammalian host to influence both homeostasis (aspects such as behavior) and susceptibility to disease. Contained within the overall hypothesis are three main thematic elements: the species composition of the microbial organ influences host homeostasis and disease susceptibility; the host's nervous system influences the species composition of the microbial organ and the microbial organ itself possesses its own nervous system. Elucidation of the mechanisms by which this evolutionary symbiosis occurs would dramatically alter current medical thought by providing a biological basis for linking these two disparate organ systems and provide a new paradigm with which to understand and design new therapeutic approaches for a range of clinical diseases. (c) 2009 Elsevier Ltd. All rights reserved.