Science continues finding evidence that the bacterial universe in our guts (the microbiome) affects our overall health, though big parts of the story still aren't clear. New research using mice adds to the intrigue by showing that social stress alters both the composition and behavior of gut bacteria, leading to self-destructive changes in the body’s immune system. If similar biological dominoes fall in humans, it may provide clues about the development of autoimmune disorders that affect an estimated one in five people.
Researchers exposed one group of mice to daily stress (rough encounters with more aggressive mice) for 10 days. Another group was left alone for the same time period. The researchers then analyzed both groups’ gut microbiota and found differences in their bacterial compositions, with the biggest changes in two particular types: bilophila and dehalobacterium. Both types have been linked with autoimmune disorders in humans, notably multiple sclerosis.
The researchers followed with a genetic analysis and found genes linked to “violent traits” switched on, which, according to the study, increase growth, movement, and signaling between bacteria and host. In other words, the bacteria appear to have turned into destructive pathogens with enhanced ability to travel through the body and infect tissue.
Further analysis of the lymph nodes of the stressed mice confirmed that connection, finding high levels of pathogenic bacteria and a density of "self-reactive effector t-cells" (immune system cells) that are characteristic of autoimmune disorders.
When taken together, these results suggest that a percentage of the stressed gut bacteria in the mice became pathogenic and infected their tissue, leading to the immune system attacking the body.
"We know that there's strong crosstalk between the immune system and the microbiota," said lead study other and immunologist Orly Avni, PhD, at Bar Ilan University, in a press statement. “An important step in understanding how stress may lead to autoimmune conditions is to identify the genetic responses of bacteria.”
The researchers think similar dynamics may occur in humans and contribute to the development of autoimmune disorders like multiple sclerosis, lupus, rheumatoid arthritis, juvenile diabetes, scleroderma, and pulmonary fibrosis. And if that’s true, the stress we experience each day could be affecting us at a much deeper level than we know.
As with all research like this, it’s important to remember that mice react differently to stress than humans, and what happens in 10 days to their microbiota and immune systems isn’t identical to what occurs in the human counterparts. Running a comparable human test would be difficult for obvious reasons, so for the moment this is an instructive albeit imperfect model to draw clues from.
The National Institutes of Health estimates that more than 20 million people in the U.S. have autoimmune diseases, with more developing the conditions every year. A possible connection between rising levels of daily stress and gut bacteria would help explain the increase, although it’s too early to draw any conclusions.
For now, this is another possible way to understand the development of autoimmune conditions, and another way to view our interrelationship with the microbiome. Much more to come on every part of the story.
The study was published in mSystems, a journal of the American Society for Microbiology.