The illness affects 1.6 million people in the United States, causing abdominal pain, diarrhea, constipation, rectal bleeding and other potentially debilitating symptoms. Crohn’s disease and ulcerative colitis are the most common forms of IBD, an autoimmune condition whose development has been linked to genetic and environmental factors.

The microbes that colonize the gut are likely an important environmental factor. In most cases, these bacteria are beneficial to humans. However, certain bacteria can get through the protective layer of mucus that covers the inner lining of the gut.

Scientists have theorized that under the right conditions, such bacteria burrow their way into the gut lining, inciting immune cells to attack and harm the intestine.

But the Washington University researchers found that these bacteria aren’t the direct fighters in this conflict; instead, they produce small particles that get into the intestinal lining and prompt an aggressive immune-system response.

“You can compare these particles, which are known as vesicles, to fighter jets being released from a bacterial mothership,” said co-author Christina Hickey, MD, a clinical fellow. “Having a more accurate picture of how these jets trigger the onset of an attack should help us devise better ways to help prevent IBD symptoms.”

The findings were published in Cell Host & Microbe.