All mammals are colonized at birth by a vast number of microbes, collectively known as the
microbiota. In our studies we are addressing whether this microbial colonization influences brain development. The largest population of microbiota is found in the gastrointestinal tract, and this is also the only microbiota with a well-established, bidirectional communication with the brain. We recently found that the absence of this colonization alters key developmental processes in the perinatal mouse brain. For example, cell death (which prunes neuronal cells) and presence of microglia (the immune cells of the brain) are altered in newborn mice devoid of microbes, in comparison to mice that are conventionally colonized at birth.