in abundance and other things appear and take their place." There are no obvious reasons for these fluctuations, but there must be important factors underlying these patterns, said Relman, associate professor of medicine and of microbiology and immunology.

Six of the 14 babies had some course of antimicrobial medicine during their first year. Only one of the babies had an extremely dramatic change in the microbial community in response to the drugs. "But it was so dramatic it makes us want to look at more examples of that and try to understand generalizations of the process," Palmer said.

The researchers had one set of fraternal twins in their study, the only babies delivered by a planned caesarean section and thus without any exposure to the mother's vaginal or rectal environments. They had much lower bacterial levels than the other babies for the first week of life.

The twins also showed the most similarity in their microbial community profiles, leading to speculation that combinations of genetics and environment can shape a microbial community. "The fact that the twins were so similar gives us a glimmer of hope that it's not a completely chaotic process," said Palmer.

Although microbes' reputation for causing disease usually gets top billing, the tiny critters play a number of critical roles in health, including processing nutrients, defining host body-fat content and providing protection against invading pathogens.

Despite their significant role in health, much of their existence remains a mystery. No more than half the total number of intestinal-tract organisms are even recognized, Relman said.

This study relied heavily on the use of a DNA microarray - technology that Brown helped develop in the mid-1990s. It consists of a glass microscope slide with an orderly array of DNA spots that can give a snapshot of genetic activity in a given sample.

The microarray design for th
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