Inflammatory bowel disease increases the risk of colorectal cancer — possibly because gut inflammation promotes the growth of a bacterial strain that produces a DNA-damaging toxin.
Christian Jobin of the University of North Carolina at Chapel Hill and his team exposed mice suffering from gut inflammation, or colitis, to gut bacteria and to a carcinogen. Animals that had been colonized with the Escherichia coli strain NC101 were much more likely to develop colorectal cancer than those that had been colonized with another human gut bacterium. However, in the absence of inflammation, mice were able to protect themselves from the E. coli strain's harmful effects. The researchers pinpointed a small region of DNA in E. coli NC101 that encodes a DNA-damaging toxin. E. coli lacking this toxin induced intestinal inflammation but not cancer.
Science http://dx.doi.org/10.1126/science.1224820 (2012)
For a longer story on this research, see http://go.nature.com/tx21ez