Aside from the brain, there are more neural cells in the gut than anywhere else in the body. At the same time, the gut is host to bacteria with genetic material that outnumbers human DNA by a ratio of 100 to 1. This bacterial mass—known as the microbiome—which is capable of producing neurotransmitters and other biological molecules, is constantly engaged in two-way communication with the brain, influencing mood, emotion, and behavior.