LOS ANGELES, March 24 (Xinhuanet)-- A bacterium that lives in the human gut can adaptively shift more than a quarter of its genes when its host's diet changes, US researchers reported on Thursday.

    Allowing the bacterial species to survive rapidly changing nutrient conditions, this mechanism also helps maintain the functions and stability of the gut's complex microbial society, according to researchers at Washington University School of Medicine in St. Louis. Their paper is published in the March 25 issue of the journal Science.

    The bacterium called Bacteriodes thetaiotaomicron (B. theta) isamong the most abundant species in the human gut microbial community. It breaks down otherwise indigestible carbohydrates, such as dietary fiber, and supplies its host with nutrients while obtaining food for itself and other gut bacterial species. The complete genome sequence of B. theta was generated two years ago.

    The researchers inoculated germ-free mice, which have no intestinal bacteria, with B. theta. The mice were fed a diet high in complex carbohydrates and low in simple sugars. Ten days later,the activity of all genes in the bacterial genome was surveyed in B. theta from the mice's guts.

    The research team found that 1,237 of the bacterium's 4,779 genes were highly active compared with B. theta grown in a simple-sugar soup. The predominant group of high-activity genes were involved in the acquisition and digestion of carbohydrates.

    When a set of germ-free mice were fed a simple-sugar diet instead of a complex-carbohydrate diet and then inoculated with B.theta, the genome activity analysis showed this bacterium had adaptively switched on a different set of genes encoding surface proteins and carbohydrate-busting enzymes. This switch allowed it to bind to and digest the host-produced mucus carbohydrates.

    The dietary switch also caused B. theta to change the activity of genes that code for components of its outer surface cell capsule. This change in the face of this friendly, or symbiotic, species may be an important mechanism to avoid causing a potentially damaging or disruptive host immune response.

    The bacterium is an incredibly sophisticated and versatile diner, the researchers said. It has evolved an elaborate mechanism for sensing changes in its nutrient landscape and quickly changingits dietary preferences so that it can use whatever is most plentiful.

    Its mechanism is essential for human intestinal health, scientists noted.

    "Bacterial cells in the human gut number close to 100 trillion," said Jeffrey Gordon, a professor at Washington University who led the study. "They can be viewed as a 'microbial organ' that lives within the intestine and harvests, stores and redistributes energy from the diet."

    Because changes in the composition of this "microbial organ" may be harmful to human health, it is important to understand how gut microbes adapt to the dynamic environment of the gut and ensure the functional stability of the intestinal bioreactor, the researchers said.

    In addition, variations in the composition of gut microbial communities among different people may be an important factor that influences predisposition to obesity and obesity-related disorders such as diabetes and heart disease.

    By defining the factors that underlie the stability of the gut ecosystem, scientists may be able to develop ways to manipulate the gut's bacterial community to promote health or treat diseases.The researchers said they are assessing how the bacterial capacity for processing dietary carbohydrates varies among individuals and what influence that may have on weight. Enditem