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Add one more to the list of tumor-causing bad guys in the colon.

In some ways, it shouldn’t come as a surprise that the teeming population of bacteria living in the intestinal tract have something to do with colon cancer. After all, there are trillions of them making their home all along the colon, so it’s only natural that some, as in any population of living organisms, are helpful to digestion while others go rogue and turn cells cancerous.

Since research on this invisible world of microorganisms that live within us — called the microbiome — emerged over the past decade, scientists have known that patients with colon cancer harbor different microbial communities in their digestive tract than those without the disease. But did the changing bacterial populations trigger the cancer, or were the shifts caused by the malignant growths, which attracted a more cancer-friendly community of bugs?

To find out, researchers at the University of Michigan conducted a series of microbial-gut-swapping tests among mice. The team removed samples of the microbiome from mice that were injected with a cancer-causing agent to develop colon tumors, and transplanted them into germ-free mice with essentially sterilized colons. These mice went on to develop twice the number of tumors than animals that were transplanted with a noncancerous microbiome.

(MORE: The Good Bugs: How the Germs in Your Body Keep You Healthy)

When the mice with colon cancer were given antibiotics to control their bacterial populations, they developed fewer and smaller tumors than those who didn’t receive the bacteria-fighting medications.

Looking more closely at the differences in the microbial universe of the animals, the scientists were even able to determine which populations of bacteria were more closely associated with colon cancer. These included those in the Bacteroides, Odoribacter and Akkermansia genera, and the Prevotellaceae and Porphyromonadaceae families. What’s likely happening, the researchers speculate, is that these communities of bacteria tend to attract the body’s inflammatory factors, and the immune cells that cause inflammation may prompt healthy cells to start dividing uncontrollably into tumors. “It’s not just the microbiome, it’s not just the inflammation, it’s both,” said one of the study’s co-authors, Patrick Schloss, in a statement.

So that means that in addition to the known risk factors for colon cancer, such as red meat, alcohol and inflammatory diseases, there may actually be a living culprit responsible for contributing to at least some cases of the disease. The Michigan team is investigating the various populations of gut bacteria they encountered to better understand which combinations are most tumor-friendly, and which might be more helpful for patients to make their colons less hospitable to malignant growths. That could lead to probiotic-type treatments that could lower colon-cancer risk.

“If you can better understand what functions in the microbial community are important for protecting against tumor formation or making it worse, we can hopefully translate those results to humans to understand why people do or do not get colorectal cancer, to help develop therapeutics or dietary manipulations to reduce people’s risk,” said Schloss.