The realization of the importance of the gut microbiota in health and disease has been one of the most exciting areas in biomedical research over the past 5 years. We are teeming with microrganisms in and on our body, having >150 times more microbial genes (the microbiome) than mammalian genes. Studies are revealing how variations in the composition of the gut microbiota influence all aspects of physiology, including brain function and even behavior. In terms of neuroscience and psychiatric disease the field is still very much in its infancy, but evidence is accumulating that it has a key role. Factors that shape the bacterial landscape include being born by Caesarian delivery, not being breastfed, environment, gestational age, host genetics, exposure to infections (both maternal and infant), and antibiotic usage. Moreover, stress, especially that in early life and prenatally, can have marked effects on microbiota composition (Borre et al, 2014).

A variety of strategies have been used to investigate the role of this so-called microbiota–gut–brain axis in health and disease. Germ-free mice (mice that never have been exposed to any bacteria) have been used to show that microbiota is crucial for hypothalamic–pituitary–adrenal axis function. Moreover, these mice have widespread neurodevelopmental changes in the brain, including alterations in monoaminergic neurotransmission and behavioral changes in anxiety (Foster and McVey Neufeld, 2013). More recently, we have shown that these mice also exhibit autism-like traits, such as deficits in sociability, social cognition, and increased repetitive behaviors (Desbonnet et al, 2014). Interestingly, as in autism and other neurodevelopmental disorders, these effects are much more pronounced in males than females. Moreover, studies in germ-free mice can be expanded to enable research on the ‘humanization’ of the gut microbiota, that is, transplanting faecal microbiota from specific human conditions or from animal models. Indeed, it has been shown that behavioral traits can be transplanted between strains of mice (Collins et al, 2013).

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