We’re only starting to understand the ways that our body’s microbiome — the complex ecosystem of microorganisms that live in our gut, skin and other organs — can impact our well-being: scientists now suspect, for example, that healthy gut bacteria can bolster the metabolism and even promote mental health.

Some microorganisms can also be unhealthy. In a new paper published in the journal Science, Yale researchers describe research showing that harmful bacteria can make their way from the small intestines of mice to the lymph nodes, liver and spleen.

“These bacteria don't stay in the gut,” said Martin Kriegel, the lead author of the paper and a professor of immunobiology at Yale. “We found that they can go through the gut lining and appear in other organs.��

It was already known that live microorganisms can make their way out of the gut, but only in response to stressors like inflammatory bowel disease and chemotherapy drugs, which induce inflammation and disrupt intestinal barriers. This is the first time, Kriegel believes, that researchers have shown that living gut bacteria — in this case, a little-studied organism called E. gallinarum — can colonize other areas of the body under regular circumstances.

The finding could have far-reaching implications in medicine. Once the E. gallinarum made its way outside the mice's gut, Kriegel and his collaborators found, it triggered symptoms of autoimmune disease. It’s possible, he believes, that the findings could lead to new approaches to treating autoimmune conditions like lupus.

Everyone has gut bacteria, and they can often be helpful rather than harmful. Another paper published in Science this week found that microorganisms that are encouraged by dietary fibers can help alleviate symptoms of type 2 diabetes.