Scientists say the mouth may act as a reservoir for intestinal disease-causing bacteria that exacerbate conditions including inflammatory bowel disease (IBD), ulcerative colitis (UC), and Crohn's disease (CD). Their analysis may open a much-needed new path forward to the development of effective treatments for these illnesses. While IBD, UC and CD are linked to an imbalance in the gut microbiota, their underlying causes remain a mystery. Previous research has hinted that oral bacteria typically don't live in a healthy intestine, because they are competing with the naturally abundant resident microbial community for space. Interestingly, increased amounts of orally-originating bacteria have been reported in the gut microbiota of individuals suffering from IBD, HIV, liver cirrhosis, and colon cancer. To investigate further, Koji Atarashi and colleagues obtained saliva samples from IBD and CD patients and transplanted them into germ-free mice. The researchers observed that strains of the bacterial species Klebsiella isolated from the saliva that inhabited the gut triggered a strong T cell immune response in some of the rodents. The authors say that Klebsiella are known to colonize the intestine when the microbiome is impaired and therefore could elicit severe inflammation in a genetically susceptible host, ultimately contributing to bowel disease. They note the results could provide a therapeutic strategy to correct IBD and many other related disorders by targeting oral-derived bacteria, particularly Klebsiella. A related Perspective by Xuetao Cao et al. delves into the findings further.