Germ‐free animals have shown reduced anxiety‐like behaviour as well as changes in NMDA receptor subunits in the amygdala and increased hippocampal brain‐derived neurotrophic factor (a protein associated with neurogenesis; Neufeld, Kang, Bienenstock, & Foster, 2011) and antimicrobial administration increased hippocampal expression of brain‐derived neurotrophic factor (Bercik et al., 2011). Here, BDNF is linked to Anxiety.