Two Studies in Europe have concluded that gut microbes can affect mood and/or depression. One , a study led by Jeroen Raes, a microbiologist at the Catholic University of Leuven in Belgium, studied 1054 people. Within this group 173 people either had been diagnosed with depression or had done poorly on a quality of life survey.  The microbiomes of these people were compared to those of the other participants. Two kinds of microbes Coprococcus and Dialister were missing from the microbiomes of the depressed subjects but not from the others.

     The team then looked at data from a study done in the Netherlands in which the microbiomes of 1064 Dutch people were assessed. They found  the same two microbes missing from those participants diagnosed with depression. While it is not completely known how the two microbes influence depression, Coprococcus seems to have a biochemical pathway resulting in formation of a dopamine metabolite 3,4-dihydroxyacetic acid. Dopamine is an essential neurotransmitter in the brain. This same microbe also makes an anti inflammatory substance gamma amino butyric acid which is significant since increased inflammation is implicated in depression.

     Resolving this microbiome-brain biochemical relationship might lead to novel therapies in the future. In the meantime clinical neuroscientist Andre Schmidt of the University of Basal has started a clinical trial in which his team is assessing the mental health and microbiota of 40 depressed people before and after they received a single fecal transplant. This transplant results in replacement of the  gut microbiome with that from a donor.