Importantly, the increase in bacterial translocation, attested by measurement of 16S rRNA subunits of intestinal microbiota in the blood plasma, and in the immune response to LPS, including a rise in circulating pro-inflammatory cytokine levels and up-regulation of TLR4 signaling in peripheral mononuclear blood cells, was observed in patients with MDD [165,166,167]. This evidence concerns the gene TLR4 and major depressive disorder.