Strange but true: the brain is shaped by bacteria in the digestive tract
People may advise you to listen to your gut instincts: now research suggests that your gut may have more impact on your thoughts than you ever realized. Scientists from the Karolinska Institute in Sweden and the Genome Institute of Singapore led by Sven Pettersson recently reported in the Proceedings of the National Academy of Sciences that normal gut flora, the bacteria that inhabit our intestines, have a significant impact on brain development and subsequent adult behavior.
http://www.scientificamerican.com/article.cfm?id=the-neuroscience-of-gut