Scientists discovered that a common type of bacteria has two distinct subspecies. One of them shields tumor cells from cancer treatment.

A type of bacteria that causes dental plaque may be behind a treatment-resistant form of colorectal cancer, a study published Wednesday in the journal Nature found.

The particular bacterium, which appears to shield tumor cells from cancer-fighting drugs, was found in 50% of the tumors tested in the study. The discovery, experts say, could pave the way for new treatments and possibly new methods of screening.

Colon cancer is the second-leading cause of cancer deaths in the United States and is expected to kill more than 53,000 people in the nation in 2024, according to the American Cancer Society.