In the past few years, it has become increasingly evident that resident gut bacteria are an important contributor to healthy metabolism and there is significant evidence linking altered composition of the gut microbiota and metabolic disorders such as obesity and depression. Preclinical work in animals have reported associations between alterations of the gut microbiome (the community of microorganism that live in the human gut) and anxiety-like behaviour, depressive-like symptoms and stress responsiveness. In line with these preclinical findings, a recent study found an altered composition of faecal microbiota in patients with MDD. Most notably, the MDD group had increased levels of Enterobacteriaceae and Alistipes but reduced levels of Faecalibacterium, which was negatively correlated with the severity of depressive symptoms.