The B-G-M axis has been studied primarily under four conditions: 1. germ free intestines, 2. infection or bacterial introduction, 3. antibiotic exposure, and 4. probiotic treatment. Germ free mice exhibit anxiety-like behavior accompanied by decreased N-metyl-D-aspartate receptor subunit mRNA expression in the amygdala and increased BDNF expression and decreased serotonin receptor 1A expression in the dentate granule layer of the hippocampus. (12) The brain is aware of the introduction of pathogenic microbes into the GI tract and this results in brain stem nuclei becoming activated and in some instances, associated with the development of anxiety-like behavior. (8) This occurs within hours after introduction of pathogens at subclinical thresholds. (8) Even minute doses of microbes that do not trigger an immune response are capable of influencing neurotransmission in the paraventricular hypothalamus, the central nucleus of the amygdale, and the bed nucleus of the stria terminalis, three regions involved in the processing of emotions related to anxiety and mood. (13) Citerobacter rodentium (CR) is a bacteria that has been evaluated in a number of studies involving mice. CR-infected mice demonstrated anxious-like behavior at 7-8 hours. (14) CR has been demonstrated to induce anxiety-like behavior associated with no change in plasma levels of IFN-γ, TNF-α, or IL-12 but with evidence of increased vagal transmission without mucosal inflammation. (15) Additional work has also demonstrated that Helicobacter pylori infected mice exhibit anxiety-like behavior corrected by treatment with a probiotic, Bifidobacterium. Oral antibiotics have also been shown to induce anxiety-like behavior in mice. (8) This behavior change is transient and behavior normalizes once normal flora has been restored. This effect is not seen with systemic antibiotic administration. Changes in behavior are accompanied by an increase in brain-derived neurotrophic factor (BDNF) in the hippocampus and amygdala. (8) Lastly, clinical evidence is mounting to support a role for probiotics in reducing anxiety and the stress response as well as improving mood in patients with IBS and those with chronic fatigue. (14) Probiotics have been shown to reduce anxiety in rats and to have beneficial psychological effects with a decrease in serum cortisol in humans. (14) Lactobacillus reuteri has been shown to decrease anxiety and the stress-induced increase in corticosterone in mice. (A) In addition, there are a number of other studies demonstrating the effects of probiotics on anxiety in humans. (8,9,13)