Even when you think you're alone, you're not. Several trillion bacteria tag along within the intestines of a typical person or other mammal.
While researchers have long known that these bacteria serve beneficial functions for their hosts, such as producing vitamins and breaking down nutrients (SN: 5/31/03, p. 344: Gut Check), it hasn't been clear why the host's immune system doesn't attack the microbes as foreign invaders. A new study suggests that these bacteria disguise themselves with sugar molecules originating in their host's intestinal cells.
Many scientists have theorized that gut bacteria use such molecular mimicry to avoid immune system detection. However, the researchers lacked convincing data. Seeking new evidence, Laurie E. Comstock and her colleagues at Harvard Medical School in Boston examined Bacteroides fragilis, a common gut bacterium.