In this sense, the microbiota is progressively regarded to be a key regulator even of neurological functions, since it may directly influence the level of brain deriving growth factor (BDNF) in mice [33] and of glial cell line-derived neurotrophic factor (GDNF) in specific brain areas [41]; moreover, on the other hand, its depletion is at the basis of many neurological or psychiatric disorders [42]. This evidence concerns the gene GDNF and psychiatric disorder.