Bacteria that make their home in the human gut cloak themselves in sugar taken from the intestine wall to hide from the immune system, U.S. researchers reported on Thursday.

The study, published in the journal Science, helps explain how people and animals tolerate trillions of bacteria in their digestive systems.

It has long been known that our guts are full of bacteria, many of which help digest food.

“It is estimated that the bacteria are about 30 percent of the stool,” said study leader Laurie Comstock of Boston’s Brigham and Women’s Hospital and Harvard Medical School.

“There about a trillion per gram of colonic content,” she added in a telephone interview.

Her team was trying to figure out how all these bacteria manage to survive without being attacked by patrolling immune system cells.

They studied in particular a group, or genus, of bacteria called Bacteroides, which help digest plant matter in mice, humans and other mammals.

These bacteria are coated in a sugar called fucose, which is not native to the bacteria. Further checks showed they are stripping it off the insides of the intestines, Comstock’s team reported.

“What we found is a pathway that helps them take this fucose molecule that they have scavenged from the host and display it on their own surface,” she said.

And other studies have shown these bacteria can cause the host animal’s intestine to produce the fucose, she said.

Although the research was done in mice, similar species of bacteria found in human colons also cover themselves in fucose, Comstock said. “We have looked at some human poop,” she said.

In mice, bacteria bred to lack the fucose die out fairly quickly, so the disguise is clearly a key to survival, Comstock said.