Gut microbes are capable of producing most neurotransmitters found in the human brain. While these neurotransmitters primarily act locally in the gut, modulating the enteric nervous system, evidence is now accumulating to support the view that gut microbes through multiple mechanisms can influence central neurochemistry and behavior. This has been described as a fundamental paradigm shift in neuroscience. Bifidobacteria for example can produce and increase plasma levels of the serotonin precursor tryptophan, which is fundamental in regulating mood, appetite and gastrointestinal function. Certain Lactobacilli have been shown to produce gamma-aminobutyric acid (GABA) and to alter brain GABA receptor expression and behavior. IBS is regarded as the prototypic disorder of the brain-gut-microbiota axis which can be responsive to probiotic therapy. Recently, the concept of a psychobiotic has been introduced in the literature. A psychobiotic is a bacteria which when ingested in adequate amounts can have a positive mental health benefit. Translational studies indicate that certain bacteria may impact upon stress responses and cognitive functioning. Manipulating the gut microbiota with psychobiotics, prebiotics or even antibiotics offers a novel approach to altering brain function and treating gut-brain axis disorders such as depression and autism.