By Gina Kolata

New York Times News Service

Two types of bacteria commonly found in the gut work together to fuel the growth of colon tumors, researchers reported 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 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.

An estimated 50,000 Americans are expected to die of colon cancer in 2018. The new study focused on the earliest stages of the disease.

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 tumors.

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.

For most who carry them, it is not clear the bacteria would ever be a problem, said Dr. Eric Pamer, an infectious disease specialist at Memorial Sloan Kettering Cancer Center in New York.

The bacteria seem to be able to induce colon cancer, or to take precancerous cells and drive them faster toward cancer, said Dr. Drew Pardoll, director of Johns Hopkins’ Bloomberg-Kimmel Institute for Cancer Immunotherapy and an author of the new paper.

“Bacteria are there at the very earliest stages,” said Dr. Matthew Meyerson, director of cancer genomics at the Dana-Farber Cancer Institute in Boston. “This is a big step forward in understanding what is the role of the microbiome in colon cancer.”

The study’s lead researcher, Dr. Cynthia Sears, an infectious disease specialist at the Bloomberg-Kimmel Institute, did not expect to be investigating colon cancer.

She and her colleagues were looking at a toxin produced by B. fragilis. It was a familiar poison, a common cause of diarrhea.

In petri dishes, however, the toxin damages DNA and triggers cancerous changes in the epithelial cells that give rise to colon cancer. When the investigators put these toxin-secreting bacteria into mice, the animals’ colons became spotted with tumors.

So the scientists decided to investigate a broader question: Were any bacteria linked to human colon cancer and, if so, which ones?

Now Sears is trying to find out whether the bacteria always, or sometimes, are drivers of colon cancer in the general population.

Working with nearby community medical practices, she is examining colon tissue from every patient having a colonoscopy. She hopes to review 2,000 pathology samples. But, she and others warn, it is premature to think of using the current results to protect people from cancer.

“You could try to eliminate the bugs,” Jobin said. “That is easy to say but hard to do. Antibiotics will probably do more damage than good.”

The bacteria described in the new study are not the first implicated in colon cancer. Researchers have discovered a different type, fusobacteria, in more advanced colon cancer.