Two types of bacteria commonly found in the gut work together to fuel the growth of colon tumours, researchers reported on Thursday.
Their study, published in the journal Science, describes what may be a hidden cause of colon cancer, the third most common cancer in the United States. The research also adds to growing evidence that gut bacteria modify the body’s immune system in unexpected and sometimes deadly ways.
The findings suggest that certain preventive strategies may be effective in the future, like looking for the bacteria in the colons of people getting colonoscopies.
If the microbes are present, the patients might warrant more frequent screening; eventually, people at high risk for colon cancer may be vaccinated against at least one of the bacterial strains.
“I can’t guarantee you these bacteria will be the holy grail of colon cancer, but they should be high on the list” of possible culprits, said Christian Jobin, a professor of medicine at the University of Florida who studies bacteria in the gastrointestinal tract.

Two types of bacteria, Bacteroides fragilis and a strain of E. coli, can pierce a mucus shield that lines the colon and normally blocks invaders from entering, the researchers found. Once past the protective layer, the bacteria grow into a long, thin film, covering the intestinal lining with colonies of the microbes.
E. coli then releases a toxin that damages DNA of colon cells, while B. fragilis produces another poison that both damages DNA and inflames the cells. Together they enhance the growth of tumours.
Not everyone carries the two types of bacteria in their colon. Those who do seem to pick up microbes in childhood, where they simply become part of the diverse mass of bacteria in the intestinal tract — the so-called microbiome.

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