Trillions of bacteria and other microorganisms are known to dwell happily in the human body. But exactly how they are able to live their days in obscurity, unbothered by the immune system, is unknown. Now scientists have some clues.

In experiments with the intestinal bacteria known as Bacteroides fragilis, the researchers found that gut flora may be able to change their surfaces to avoid detection by the immune system.

The bacteria produce at least eight different sugars, or polysaccharides, on their surfaces. And this shift of outerwear could perhaps act as a disguise.

These eight distinct polysaccharides are the most yet seen on any type of bacteria.

Exactly how the immune system normally reacts to the body's population of microorganisms, or flora, is unclear. In fact, the whole area is a "huge black box."

Clearly, these flora do good things, such as competing with invading microbes that cause disease. But they can occasionally get involved in the disease process. For instance, if there is an injury to the intestines that causes flora to leak they can cause infection.

It is possible that if the immune system were primed to attack Bacteroides organisms with a particular polysaccharide, the microbes could put on a different surface sugar to allude detection.

The organisms are obviously quite good at staying alive since they withstand invading germs, bacteria-killing antibiotics and various "environmental" shifts such as dietary changes. This study suggests that an occasional change of wardrobe is one way they do it.

Nature November 29, 2001;414:555-558