Gut check: the bacteria in your intestines are welcome guests.

New York and London are famous for both their congestion and the diverse origins of their residents. But if you're looking for the ultimate teeming metropolis of immigrants, cheek out the large intestine. In people, some 500 to 1,000 kinds of bacteria reside in this part of the gastrointestinal (GI) tract, and these gut microbes outnumber all the cells in your body, perhaps by as much as a factor of 10.

"The density of this society is mind-boggling," says Jeffrey I. Gordon of Washington University School of Medicine in St. Louis.

It's a society overlooked by most microbiologists, who generally stick to the myriad bacteria that cause disease. Yet some scientists argue that it's shortsighted to ignore what they call the microflora living in our intestines.

"What these bacteria do definitely makes a very significant contribution to our health--or lack thereof," says Mark Schell of the University of Georgia in Athens, who studies an intestinal microbe called Bifodobacterium longum.

Shell and a few other researchers have recently begun to probe exactly what individual microbes do for or to the intestine.

Consider Bacteriodes thetaiotaomicron. Although not as well known, it's more than 1,000 times as abundant in the guts of people and mice as the extensively studied bacterium Escherichia coli. Some researchers have proposed that in return for a steady food supply, B. thetaiotaomicron breaks down indigestible complex carbohydrates into easily absorbed sugars and produces other substances, such as vitamins, that benefit its host.

There may be much more to this microbe-host relationship, however. About a decade ago, Gordon chose B. thetaiotaomicron as a prototypical germ for studying how microbes influence the GI tract. This bacterium normally becomes a predominant member of the intestinal community about the time an animal is weaned from its mother's milk. Gordon's research team has…