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Another surprise: There isn’t one core set of bacteria that perform those functions. A wide variety can do the same jobs, the researchers found.

That’s fortunate considering people carry a customized set of microbes, one that varies dramatically depending on where you live, your diet and a host of other factors. Your microbial zoos also can change, such as when taking antibiotics that kill infection-causing germs as well as good intestinal bacteria that may be replaced with different but equally effective bugs.

“We don’t all have the same bacteria although they all seem to have been organized to do the same things,” Birren said. It may be that our lifestyle and environment “induces each of us to have arrived at a solution that works for us.”

With this first snapshot of what normal looks like, studies now are under way to see how the microbes differ in people with certain diseases, in hopes of learning how to prevent or treat the illnesses.

Consider the intestinal superbug named C. difficile that people all too often catch while they’re in the hospital, and that sometimes kills. Washington University’s Tarr wants to know what mixture of gut bacteria can fend off the diarrhea-causing germ or make it more likely to infect _ so that doctors might one day know who’s more vulnerable before they enter a hospital.

Also, researchers at Baylor College of Medicine reported Wednesday that the kind of bacteria living in the vagina changes during pregnancy, perhaps to give the fetus as healthy a passage as possible. Previous research has found differences in what first bacteria babies absorb depending on whether they’re born vaginally or by C-section, a possible explanation for why cesareans raise the risk for certain infections.

All new information in some ways is humbling, because it shows how much more work is needed to understand this world within us, noted infectious disease specialist Dr. David Relman of Stanford University, who wrote a review of the project’s findings for the journal Nature.

For example, the project included mostly white volunteers who live around Houston and St. Louis. Relman said more work is needed to define a normal microbiome in people with different racial, ethnic and geographic backgrounds.

And there are many remaining questions about how these microbes interact with human genetics.

“We are essentially blind to many of the services that our microbial ecosystems provide _ and on which our health depends,” Relman wrote.

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Online:

Nature: http://www.nature.com

PLoS: http://www.ploscollections.org/hmp