Two new studies tell us an interesting story about stress, the gut and the brain.

We have a LOT of organisms in the gut. Cell for cell, we’re largely made up of bacteria. In fact, single-celled organisms, mostly bacteria, outnumber our own cells 10 to 1. Most of these live in our gut.

Any alteration of the composition of good versus bad microorganisms in the gut —collectively known as our “microbiome”—impacts the state of our brain, making us more prone to anxiety, depression and low mood. And that lowered mood makes us more prone to feeling stressed out and reactive… which in turn further changes the composition of microorganisms in the gut…

See the cycle?

The first study appears in the journal Brain, Behavior, and Immunity. Researchers report that when we are under stress, the bacterial communities in our intestine become less diverse, allowing greater numbers of harmful bacteria to take over and party hearty.

(We’ve pretty much known this for a while, after all, disorders of the gut such as irritable bowel and inflammatory bowel diseases are known to worsen during times of stress. But now we have the science to back up that clinical observation.)

And that leads to the second study I wanted to talk about. It turns out that when bad bacteria are partying in our gut, it not only lowers our overall immunity, it lowers our overall mood. A sophisticated neural network transmits messages from those trillions of bacteria to our brain, exerting a powerful influence on our state of mind.

That’s why scientists have begun to refer to our gut as “the second brain.”

The idea that bacteria teeming in the gut can affect the mind “has just catapulted onto the scene,” say study authors. Our gut bacteria manufacture about 95 percent of our body’s supply of serotonin, which significantly influences our mood. In just the last few years evidence has piled up that the gut microbiome heavily influences our neural development, brain chemistry, emotional behavior, pain perception, learning, memory, and how our stress system is prepared to respond to life’s ups and downs.

The more bad bacteria in our gut, the more anxious and moody and stressed out we feel.

The more stressed out we feel the more we tip the microbial balance in the gut allowing more bad bacteria to thrive.

Gastroenterologist Emeran Mayer, MD, director of the Center for Neurobiology of Stress at the University of California, Los Angeles, says that given the gut’s multifaceted ability to communicate with the brain “it’s almost unthinkable that the gut is not playing a critical role in mind states.”

THE LAST BEST CURE is a toolbox to  intervene in that stress feedback cycle and help rescue both the brain in our gut and the brain in our head.

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“Exposure to a social stressor alters the structure of the intestinal microbiota: Implications for stressor-induced immunomodulation” by Michael T. Bailey, Scot E. Dowd, Jeffrey D. Galley, Amy R. Hufnagle, Rebecca G. Allen and Mark Lytee; and the brief commentary on it is “The gut microbiota: A new player in the innate immune stress response?” by Monika Fleshner. The article appears in Brain, Behavior, and Immunity, Volume 25, Number 3 (March 2011), published by Elsevier.