D:        Yaël, biomedical researchers know that the brain and the gut communicate by neural, hormonal, and immune system links.  Some have become interested in whether the trillions of microbes that grow in our gut might influence our mental health.

Y:        Yes, but understanding whether and how microbes in the gut could influence the brain is hard.  Studies in animals might not accurately reflect human traits and symptoms, and human studies have often involved limited numbers of people.

D:        So, what are researchers doing to overcome these problems?

Y:        In twenty nineteen, a team of Belgian researchers obtained new evidence correlating gut bacteria with depression in humans. The researchers studied more than a thousand human subjects.  They used a questionnaire survey or a physician’s diagnosis to determine whether or not the subjects were depressed.  They used Dee En Aye sequencing to determine what species of bacteria were found in the um...feces of all these subjects.

D:        Yuck!  That must have been a difficult study.  What did they find?

Y:        They found a correlation between the patients’ depression and the absence of two bacteria species.

D:        Which means they don’t know whether being depressed caused the lack of the bacterial species, or the lack of the bacterial species caused the depression, or whether there’s some interaction between the two.

Y:        That’s right.  The bacteria do synthesize a breakdown product of the neurotransmitter dopamine, and while there’s evidence linking dopamine to depression, other neurotransmitters, like serotonin are also involved.