Theories about the association of imbalances in the gastro-intestinal (GI) microbiota and its impact on mood states have long existed in medicine. These theories were considered "dead" for most of the 20th century, but converging lines of evidence are now rekindling interest in a link between the GI microbiota and the central nervous system. Newer lines of investigation have made these dormant theories relating commensal microbiota and mood states not only plausible, but the focus of well-designed basic research studies (Yano et al, 2015; Hsiao et al, 2013). The microbial world influence on brain development and behavior was defined by Dr Tom Insel, the former director of the National Institutes of Mental Health as one the "great frontiers" of neuroscience in the next decade (Insel, 2012). Dr. Francis Collins, Director of the National Institutes of Health, further underscored the need for research on the impact of intestinal microbiota on brain function (Collins, 2014).