Mouth to Gut Bacteria Migration: Uncovering Why Smoking May Protect Against Inflamed Bowels

Researchers from the RIKEN Center for Integrative Medical Sciences in Japan have made a groundbreaking discovery explaining why smoking appears to have an unexpected protective effect against ulcerative colitis, a form of inflammatory bowel disease. The study shows that smoking induces metabolic changes in the gut, allowing bacteria typically found in the mouth—such as Streptococcus—to migrate and establish in the lining of the large intestine. This migration triggers a beneficial immune response that can reduce intestinal inflammation.

The team, led by Hiroshi Ohno, observed that in individuals with ulcerative colitis, smokers had higher levels of certain metabolites, like hydroquinone, which are produced as a result of smoking. These substances promote the growth of oral bacteria within the gut mucosa—a phenomenon not seen in ex-smokers. The metabolic role of hydroquinone was confirmed through experiments with mice, where its administration fostered Streptococcus growth in the gut.

Importantly, the study identified that these bacteria, particularly Streptococcus mitis, can activate helper Th1 cells, a component of the immune system. In ulcerative colitis, Th1 cells help suppress the overactive Th2 immune response responsible for inflammation. Conversely, in Crohn's disease, a different form of IBD, Th1 activation worsens inflammation, which explains why smoking increases risks for Crohn's but protects against ulcerative colitis.

The findings suggest that mimicking this bacterial migration or the metabolic effects of smoking—using prebiotics like hydroquinone or probiotic therapies containing Streptococcus mitis—could provide new treatments for ulcerative colitis without the health risks associated with smoking. This research sheds light on the complex interactions between smoking, gut bacteria, and immune responses, opening potential avenues for targeted therapies in inflammatory bowel diseases.