That gut microbes, collectively called the microbiota, influence brain development and functioning is viewed as a new paradigm in neuroscience with implications for psychiatry. These gut microbes communicate with the brain via a number of routes including the vagus nerve and the production of molecules such as short-chain fatty acids. In major depressive disorder the gut microbiota shows a significant decrease in microbial diversity which is associated with a peripheral inflammatory phenotype. Psychobiotics are bacteria which, when ingested in appropriate amounts, have positive mental health benefits. Preliminary studies with bacteria such as Bifidobacterium longum indicate anxiolytic activity in healthy volunteers.