"Two kinds of microbes, Coprococcus and Dialister, were missing from the microbiomes of the depressed subjects, but not from those with a high quality of life. ... They also found the depressed people had an increase in bacteria implicated in Crohn disease, suggesting inflammation may be at fault."

Ties in with this thread from a few months ago: https://news.ycombinator.com/item?id=18750985