Scientists have found patients with the disease had fewer "friendly" bugs and more harmful bacteria than healthy people.
Study leader Dr Jiyoung Ahn, from New York University Cancer Institute, said: "Our findings are important because identification of these microbes may open the door for colorectal cancer prevention and treatment."
Gut bacteria play an important role in regulating digestion and inflammation. Increasing evidence links gut microbes to colorectal, or bowel, cancer.
The researchers compared the DNA of intestinal microbes in 141 bowel cancer patients and healthy volunteers.
They found patient samples had larger populations of Fusobacteria - a type of bug commonly found in the mouth and gastro-intestinal tract associated with gut inflammation.
Bowel cancer patients were more likely to be lacking Clostridia, a class of "friendly" gut bacteria that help digest dietary fibre and carbohydrates.