It is not a new concept that gut bacteria affects a person’s health. But this article published in The Atlantic explains how they may even affect the human brain. Some researchers believe that the microbiome may play a role in regulating how people think and feel. Scientists have found evidence that this community of bacteria (trillions of cells that together weigh between one and three pounds) could play a crucial role in autism, anxiety, depression, and other disorders.
Much of the most intriguing work has been done on autism. For years, it has been noted that about 75 percent of people with autism also have some gastrointestinal abnormality, like digestive issues or food allergies. This has prompted scientists to search for potential connections between the gut microbiome and autism; recent studies find that autistic people’s microbiome differs significantly from those of control groups. Caltech microbiologist Sarkis Mazmanian specifically focuses on a species called Bacteroides fragilis, which is seen in smaller quantities in some children with autism. Mazmanian and several colleagues fed B. fragilis from humans to mice with symptoms similar to autism. The treatment altered the makeup of the animals’ microbiome, and more importantly, improved their behavior: They became less anxious and communicated more with other mice.
Perhaps the most well-known human study was done by Emeran Mayer, a gastroenterologist at UCLA. He recruited 25 subjects (all healthy women) for four weeks. He had 12 of them eat a cup of commercially available yogurt twice a day, while the rest didn’t. Yogurt is a probiotic, meaning it contains live bacteria. In this case it contained four species: bifidobacterium, streptococcus, lactococcus, and lactobacillus. Before and after the study, subjects were given brain scans to gauge their response to a series of images of facial expressions—happiness, sadness, anger, and so on.
To Mayer’s surprise, the results showed significant differences between the two groups. The yogurt eaters reacted more calmly to the images than the control group. “The contrast was clear,” says Mayer. “This was not what we expected, that eating a yogurt twice a day for a few weeks would do something to your brain.” He thinks the bacteria in the yogurt changed the makeup of the subjects’ gut microbes, and that this led to the production of compounds that modified brain chemistry.
As scientists learn more about how the gut-brain microbial network operates, they think it could be manipulated to treat psychiatric disorders. And because these microbes have eons of experience modifying our brains, they are likely to be more precise and subtle than current pharmacological approaches, which could mean fewer side effects. “I think these microbes will have a real effect on how we treat these disorders,” neuroscientist John Cryan says. “This is a whole new way to modulate brain function.”