GLUL and major depressive disorder: The relative abundance of five bacterial communities is elevated in patients with major depressive disorder, as detailed below: Alistipes, an indole-positive organism, can reduce the bioavailability of serotonin and metabolizes glutamate to γ-aminobutyric acid (GABA) through the expression of glutamate decarboxylase, and an increase in its abundance may disrupt the function of the gut-brain axis (21).