BDNF and anxiety disorder: This preclinical work demonstrated the impact of the gut microbiome on many physiological pathways involved in the pathogenesis of anxiety disorders, including hypothalamic–pituitary–adrenal (HPA) axis responsivity, immune modulation, tryptophan-kynurenine metabolism, vagal nerve communication, brain-derived neurotrophic factor (BDNF) expression, neurogenesis and myelination, microglial function and neurotransmitter production [7–9].