From Scientific American: http://www.scientificamerican.com/article.cfm?id=the-neuroscience-of-gut&page=2
Regardless of how these intestinal “guests” exert their influence, these studies suggest that brain-directed behaviors, which influence the manner in which animals interact with the external world, may be deeply influenced by that animal’s relationship with the microbial organisms living in its gut.
Image is from here: Giant Microbes