In an interesting little study looking closely at inflammatory bowel disease mechanisms, mice were stressed and observed for their levels of stress related hormones. The hormones elevated during stress appeared to reduce the healthy gut flora and provoke inflammation of the gut lining, akin to inflammatory bowel disease. Treating the mice with supplemental probiotics, compensating for the ones reduced by stress, resulted in less inflammation in the bowel's lining.
This laboratory observation in mice correlates well with clinical experience: stress is well known for triggering irritable and even inflammatory bowel reactions in sensitive people. Probiotic supplementation can prevent or reverse those changes, without necessarily changing the stressors themselves.