Scientists have identified a combination of bacteria that appears to increase the risk of colon cancer. Through a series of experiments in mice, they were able to pinpoint ways in which the two species of bacteria promote inflammation and break down the mucus layer of the colon. Christine M. Dejea and colleagues began by analyzing tissue samples from patients with a genetic condition associated with increased risk of developing precancerous polyps in the colon, called familial adenomatous polyposis (FAP). Unlike samples from patients with sporadic polyps, FAP samples contained patches of bacteria in which Escherichia Coli and Bacteroides fragilis (ETBF) were particularly prevalent, a result confirmed through a bigger dataset of samples from FAP patients. Intriguingly, E.coli and ETBF express genes associated with increased DNA damage and tumorigenesis, respectively. When the researchers implanted these bacteria from FAP patients into mice, they found that mice with only one species developed few tumors, whereas mice that were colonized with both bacterial species were substantially more prone to invasive cancer and death. Through a series of experiments, Dejea et al. found that ETBF alters the microenvironment of the gut by depleting mucus and inducing an inflammatory response, which helps E.coli colonize the gut as well. Based on these results, the authors suggest that eliminating these bacteria from the mucus lining of FAP patients early in life may be beneficial.