There is an emerging body of evidence that the gut microbiome also affects cognition, behavior, and brain health. This is cutting-edge stuff and the story is still being written. We already know that serotonin is an important neuroregulator of mood, memory, and anxiety. It turns out that 90 percent of the serotonin in the body resides in the gut, and it is manufactured there by bacteria such as Candida, Streptococcus, Escherichia, and Enterococcus.