How gut bacteria escape detection

Bacteria play a key role in digestion

Bacteria living in the human intestine escape detection by the immune system by disguising themselves as gut cells, researchers say.

A US team examined bacteria from the genus Bacteroides - the most commonly found bacteria in the human gut.

They found the bacteria wrap themselves in a sugar substance derived from molecules taken from the surface of cells in the gut.

Details of the Harvard Medical School research are in the journal Science.

It is extremely important that the resident bacteria in our intestine do not generate a deleterious immune response Dr Laurie Comstock

This form of molecular mimicry may help to explain how humans tolerate the presence of billions of bacteria in the gut without launching an immune system attack.

The Harvard team found that Bacteroides bacteria coat themselves with a form of fucose, a molecule which is abundant on the surface of intestinal epithelial cells.

Previous research has shown that Bacteroides can stimulate intestinal epithelial cells to produce molecules such as fucose.

It now seems that the bacteria are then able to incorporate this molecule in a slightly modified form directly into their own cells.

Digestive role

Bacteroides play a crucial role in the digestive process, helping to break down food products and supplying some vitamins and other nutrients that we cannot make ourselves.

Researcher Dr Laurie Comstock told the BBC News website: "Compared to the wealth of knowledge regarding mechanisms used by bacterial pathogens to cause disease, relatively little is known about how the trillions of bacteria in the mammalian intestine establish and maintain beneficial relationships with the host.

"It is extremely important that the resident bacteria in our intestine do not generate a deleterious immune response.

"Bacteroides are able to degrade a great variety of plant polysaccharides so that the host is able to use them for energy.

"Many of these plant polysaccharides could not be used by the host without the aid of the bacteria."

Dr Alastair Forbes, a consultant gastroenterologist at St Mark's Hospital, London, said the findings were interesting.

He said it was possible they could be used in the long-term to develop new forms of probiotic treatment for gastrointestinal disorders.

Probiotics, preparations containing beneficial bacteria, have already been used to treat conditions such as ulcerative colitis and pouchitis.

"The use of probiotics has had some success, but their use is made difficult by the fact that the bugs tend not to persist in the gut, and treatments have to be given week after week," Dr Forbes said.

"If there was some way of modifying bacteria so they could continue to infect the gut, but remain safe, that would be useful."

Dr Forbes said scientists were also working on prebiotic treatments, which provide nutrients to encourage the growth of certain bacteria.

However, he said knowledge about the bacteria which live in the gut was still relatively limited.

It is estimated that only about 20-30% of the bacteria that inhibit the gut have been identified.