The human gut is home to an astonishing number of microorganisms — 500 to 1000 bacterial species. Many of these live at peace with their hosts, providing essential functions for metabolism and the immune system, and they offer an attractive model to study symbiotic evolution. In the March 28 Science, two groups report genome sequences of members of the human intestinal community.

Jian Xu and colleagues at the Washington University School of Medicine, St Louis, US, sequenced the complete 6.26 Mb genome of the Gram-negative anaerobe Bacteroides thetaiotaomicron, a dominant gut microbe (Science, 299:2074-2076, March 28, 2003). They predicted a proteome of 4779 proteins over half of which have well-characterized functions. It has an expansion of genes involved in polysaccharide uptake and degradation, as well as environment-sensing systems, and more glycosylhydrolases than any other bacteria — indicative of its role in degrading indigestible dietary polysaccharides....