By exposing female mice to the gut bacteria of a healthy adult male, researchers were able to prevent the females from developing type 1 diabetes, an autoimmune disorder. The study, published in Science on Thursday, also shows that the treatment changed the levels of testosterone in female mice, which typically develop type 1 diabetes at a higher rate than their male counterparts.

The females of the strain of mice used in the study have a 90 percent chance of developing diabetes, says senior author Jayne Danska, an immunologist and geneticist at the Hospital for Sick Children and the University of Toronto. But by transferring the normal bacteria from the intestines of adult male mice to young female mice, the researchers were able to reduce the rate of disease to 25 percent. The changes in testosterone levels in the female mice did not reach typical male levels, but the authors show that only with active testosterone signaling did the female mice gain protection against diabetes, demonstrating that the microbe-induced increase in testosterone was critical to changing their disease rate.   

Researchers are increasingly cataloging the microbes in our body (see “Researchers Catalogue Your Microbial Zoo”) and figuring out how disease can arise when these communities are off-kilter. A microbiome connection to heart disease, obesity, and other conditions has been suggested by different groups, and some doctors have even begun to treat patients with gut bacteria transfers (see “Transplanting Gut Microbes to Treat Disease”). Today’s study suggests that changes in the gut microbial community have the potential to treat autoimmune diseases, as well.

The study by Danska and colleagues suggests a “symbiotic feedback loop” in which the sex of the animal affects the gut bug community and the gut bug community reinforces sex hormone levels. “So often, bacteria are viewed as the enemy,” says Danska. “But in some cases, microbes help protect us from pathogens and help us develop healthy [hormone] systems and healthy metabolisms.”

Male mice of the same genetic background have a 40 percent chance of developing diabetes. In humans, type 1 diabetes does not have such a large gender gap, but many other autoimmune diseases do. For example, women are much more likely to develop multiple sclerosis, rheumatoid arthritis, and autoimmunity against the thyroid, says Danska.