Are you 100 percent human? Not exactly. Bacterial cells in our bodies outnumber Homo sapiens cells by 10 to one, thriving and dividing in our mouth, gut, reproductive tract, skin, ears, and nose (and—as a new study shows—on a variety of surfaces in the New York City subway system). The microbiome, as this teeming population of more than 100 trillion microbes is known, provides many benefits, helping us to digest food and outcompeting “bad” bacteria that can cause disease.
Another recent study, published in the journal Frontiers in Psychiatry, suggests that stress can change the composition of these communities, which may in turn affect early brain development. “Changes in the microbiome may influence human behaviors such as anxiety, [and] human experience, such as stress, may change a person’s gut microbiome,” the study’s lead author, Tamar L. Gur, explained to me via email. “This idea is now being brought to infant neurodevelopment, especially since the uterus, which was long thought to be a sterile environment, has recently shown to have a microbial community harbored in the placenta.”
Indeed, research published last May showed that the placenta shelters an array of helpful bacteria matching those in our mouths. These oral bacteria may drift into the bloodstream and “seed” the baby’s microbiome in utero. Certain variations in the placental microbiome are also associated with preterm birth.
“It is likely that the microbiome from the mother, and therefore the mother’s experience (stress, environment, diet), may be the main force shaping the developing infant’s microbiome,” says Gur, an assistant professor of psychiatry, neuroscience, obstetrics and gynecology at the Wexner Medical Center at Ohio State University. Studies in mice and monkeys, and in humans taking school exams, have shown that stress can reduce populations of Lactobacillus, a typical inhabitant of the human gut. This in turn may affect levels of certain types of immune factors in the blood, which can bear upon the function of the central nervous system. Bacteria in the digestive system also break down nutrients, generating small molecules like amino acids that can activate nerves or even switch genes on and off, a process that in early development can cause long-term behavioral change.
Studies of mice lacking a microbiome have shown these animals to possess decreased levels of an important brain protein called brain-derived neurotrophic factor, which is important in neurodevelopment. Although these are animal studies, and “to date this mechanism has not been described in humans,” Gur says, it is known that pregnant women exposed to farm animals or household pets are less likely to bear children with allergies or asthma.
“While this does not involve the brain, it suggests that this type of finding may be extended to psychiatric diseases as well, since we know that there is an interaction between the microbiome and the brain,” Gur says. The researchers write in their article, “The ability of the microbiome to impact the developing central nervous system … is an exciting concept, because it is susceptible to targeting with pre- and probiotics” that can boost levels of beneficial bacteria.