A mysterious bacterium found in up to half of all colon tumors also travels with the cancer as it spreads, researchers reported on Thursday.

Whether the bacterium, called Fusobacterium nucleatum, actually plays a role in causing or spurring the growth of cancer is not known. But the new study, published in the journal Science, also shows that an antibiotic that squelches this organism slows the growth of cancer cells in mice.

Scientists are increasingly suspicious that there may be a link: another type of bacteria has been discovered in pancreatic cancer cells. In both types of cancer, most tumors host bacteria; however, only a small proportion of the cells in any single tumor are infected.

“The whole idea of bacteria in tumors is fascinating and unexpected,” said Dr. Bert Vogelstein, a colon cancer researcher at Johns Hopkins.