Gut bacteria (or gut microbiota) is as hip in the world of science as a Brooklyn address is in the world of New York real estate. Scientists use it to explain all manner of human conditions and biological effects, including obesity and immunity, and, most recently, they have begun to explore its effects on cognitive processes and feelings. In a recent paper, Mexican scientists take this one step further and suggest that this gut-brain axis, particularly the ability of the gut microbiota to communicate with the brain, may even modulate social behavior.

“The similarity of microbial communities across individuals is an index of the strength of their social bonds,” wrote the authors. “In our opinion, testing this hypothesis may add an important analytical tool to research focused on how social bonds … translate into cooperation and cohesion at the group-level, an approach that could ultimately shed light on the origin and evolution of sociality.”

Bidirectional Communication

The human intestine harbors nearly 100 trillion bacteria. They contribute to digestion by helping to break down complex carbohydrates and starches, and they are crucial to the immune system as well. Scientists once theorized that infants are born with sterile guts and only became colonized with microbiota within the first few days of life, but now it is known that infants begin acquiring intestinal flora when they ingest amniotic fluid in the womb. Nevertheless, while passing through the birth canal, an infant swallows bacteria from its mother, and this sudden infusion of bacteria is key. The initial colonization determines what follows: The existing bacteria influences the genetic expression of cells within the digestive tract, and by doing this, they regulate the environment, creating one that is most favorable to themselves and less favorable to other bacteria introduced later.

Meanwhile, scientists have also explored the pathways of communication between the gut microbiota and the central nervous system. Previously, many scientists favored theories in which the head crucially influenced the gut — anxiety causing a stomach ache, for instance — but more recently, scientists are giving credence to the ways in which gastrointestinal tract bacteria activate neural pathways and central nervous system (CNS) signaling systems. Scientists, including a team of Canadian researchers, are increasingly beginning to understand that communication does not just flow from the top down, but from the bottom up as well. In other words, communication along the gut-brain axis is a proverbial two-way street with microbiota influencing the nervous system as much as the nervous system influences the bacteria living within our digestive tracts. “Gut feelings,” then, have raw truth from a scientific perspective.

A team of researchers led by Augusto J. Montiel-Castro suggest that human microbiota constitute a diverse and dynamic ecosystem that is in a “mutualistic relationship with its host.” They believe that gut microbiota not only have the ability to communicate with the brain but that these bacteria also modulate behavior. In fact, Montiel-Castro hypothesizes that humans have developed cultural strategies intended "to select, transfer, and eliminate microorganisms" within a given microbiota profile. The formation of social-borders, then, is intended to limit the extent of microbial transmission. Choices about who we kiss, share food with, and touch, have everything to do with a desire to share gut microbiota and so, too, a desire to extend and share collective immunity against pathogens. Welcome to the neighborhood!

Yet, because communication along the gut-brain axis is bidirectional, the researchers suggest that this social relationship might possibly work the other way around, with behavior being an expression of our intestinal tracts. Collectivism and cooperation may be the natural outcome when society members have similar gut microbiota, whereas individualism is quite possibly a general tendency in which members of societies have less similar microbial communities residing in their guts.

Sources: Montiel-Castro AJ, Gonzalez-Cervantes RM, Bravo-Ruiseco G, Pacheco-Lopez G. The microbiota-gut-brain axis: neurobehavioral correlates, health and sociality. Integrative Neuroscience. 2013.

Foster JA, McVey Neufeld K-A. Gut–brain axis: how the microbiome influences anxiety and depression. Trends in Neuroscience. 2013.