We humans have a second brain. The second one, called our enteric nervous system, consists of some 100 million neurons that are embedded in the walls of the long tube of our gut, which starts at the esophagus and ends at the anus. It measures approximately nine meters long, deeper than most swimming pools.

As important as the neurons in the gut is the kind of bacteria found there. Our body is a dwelling place for about 100 trillion bacteria and other microbes, collectively known as our microbiome. They do many important things: break down our food, fight off infection, and boost our immune system. However, scientists are finding that they may do even more than that, and have an important role in our mental health. In fact, the burgeoning field of psychobiotics may prove to be a new treatment for those with chronic depression, and especially for those who suffer from gastrointestinal issues alongside depression and anxiety.

John F. Cryan, PhD, a neuropharmacologist and microbiome expert from the University College Cork in Ireland, is one of the scientists at the forefront of exploring the link between gut and brain health. He works closely with gastroenterologists, microbiologists, and psychiatrists to study the effects of gut bacteria on the brain. His studies on mice are fascinating, and show us how gut bacteria can alter the biochemistry of our brain (the one held up by our necks).

Dr. Cryan and his colleagues discovered that when mice are bred in sterile conditions — lacking of beneficial bacteria — they do not interact with other mice and behave with social awkwardness, much like I do at a PTA meeting. Also, when they disrupted the microbiome, the mice mimicked human anxiety, depression, and autism. Cut off the good stuff, and these guys aren’t happy.

Cryan began as a neuroscientist and studied mostly the brain; however, after seeing how patients with comorbid illnesses were being tossed from one specialist to another — gastroenterologists referring to psychiatrists and vice versa — he wanted to explore the link between our intestines and our noggin in order to improve healthcare. His studies provide the data to support new forms of treatment and encourage other neuroscientists to venture below the neck.

Sarkis Mazmanian, PhD, is another pioneer in this field. A microbiologist at the California Institute of Technology in Pasadena, he was recently interviewed for the journal Nature. “The field [of neuroscience] is going to another level of sophistication,” he said. “Hopefully this will shift this image that there’s too much commercial interest and data from too few labs.”

Dr. Mazmanian did his own study in 2013 that found mice with some features of autism had much lower levels of a common gut bacterium called Bacteroides fragilis than did normal mice. They were stressed, antisocial, and had the same gastrointestinal symptoms often found in autism. Interestingly enough, when the scientists fed the mice B. fragilis, they reversed their symptoms.

If you are skeptical of mice experiments, there’s also this. Many studies have indicated that (human) babies born by C-section have an increased risk for developing allergies, asthma, diabetes, and autism. That was the same as mice born of C-sections. But they also were more anxious and depressed. Why? They don’t receive the critical exposures to a mother’s vaginal microbes when they are born.

What does this mean in terms of treatment?

In one of Cryan’s studies, two varieties of Bifidobacterium produced by his lab were more effective than escitalopram (Lexapro) at treating anxious and depressed behavior in a lab mouse strain known for pathological anxiety.

I began taking a probiotic last year and I do think it has helped my mood. Like the lab mice, I feel more resilient. I think it’s especially important for people who have been on a lot of antibiotics in their past, as I was, or have recently had major surgery. Looking back, I think my appendix rupturing and the ensuing appendectomy affected my mental health more than I ever considered. The probiotic treatment has helped heal that.

The more I ask people about the link between their gastrointestinal problems and mood disruptions, the more convinced I am of how the two brains work together.

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