Evidence has mounted that the gut microbiome can influence neural development, brain chemistry and a wide range of behavioral phenomena, including emotional behavior, pain perception and how the stress system responds.
With a sophisticated neural network transmitting messages from trillions of bacteria, the brain in your gut exerts a powerful influence over the one in your head, new research suggests.
If aliens were to swoop in from outer space and squeeze a human down to see what we’re made of, they would come to the conclusion that cell for cell, we’re mostly bacteria. In fact, single-celled organisms—mostly bacteria—outnumber our own cells 10 to one, and most of them make their home in the gut. The gut, in turn, has evolved a stunningly complex neural network capable of leveraging this bacterial ecosystem for the sake of both physical and psychological well-being.
The idea that bacteria teeming in the gut—collectively known as the microbiome—can affect not only the gut, but also the mind, “has just catapulted onto the scene,” says neuroimmunologist John Bienenstock, MD, of McMaster University in Hamilton, Ontario. In just the last few years, evidence has mounted from studies in rodents that the gut microbiome can influence neural development, brain chemistry and a wide range of behavioral phenomena, including emotional behavior, pain perception and how the stress system responds.
Research has found, for example, that tweaking the balance between beneficial and disease-causing bacteria in an animal’s gut can alter its brain chemistry and lead it to become either more bold or more anxious. The brain can also exert a powerful influence on gut bacteria; as many studies have shown, even mild stress can tip the microbial balance in the gut, making the host more vulnerable to infectious disease and triggering a cascade of molecular reactions that feed back to the central nervous system.
Such findings offer the tantalizing possibility of using beneficial, or probiotic, bacteria to treat mood and anxiety disorders—either by administering beneficial microbes themselves or by developing drugs that mimic their metabolic functions. The new research also hints at new ways of managing chronic gastrointestinal (GI) disorders that are commonly accompanied by anxiety and depression, and that also appear to involve abnormal gut microbiota.
As exciting as these investigations may be, research on how gut bacteria affect psychological well-being in humans is still in its infancy. For one, the studies have been almost entirely limited to rodents. Second, researchers have only begun to probe how such effects occur. Finally, correcting microbial imbalances to treat disease requires first defining what constitutes a healthy gut microbiome—something that scientists are still trying to understand.
"We’re just scraping the surface," says McMaster University gastroenterologist Premysl Bercik, MD. "Definitely the animal data suggest that bacteria can have profound effects on behavior and brain biochemistry, probably through multiple pathways." Untangling those biological processes and learning how to apply that knowledge to boost human psychological health will take many years.