November 16, 2009

Wide Variety of Bacteria Mapped Across the Human Body

an image of rod-shaped bacteria Electron micrograph of bacteria. David Gregory and Debbie Marshall, all rights reserved by Wellcome Image

By analyzing bacterial communities in and on several people, scientists have begun to create an atlas of bacterial diversity that documents the different types of microbes that thrive in distinct regions of the human body. This research sets the stage for determining how changes in bacterial communities help to cause or prevent disease.

Our bodies play host to a wide variety of microbes, called the human microbiota, that outnumber our own cells by about 10 to 1. Many of these microbes help us stay healthy—for instance by aiding digestion or crowding out disease-causing microbes. But details about how microbial communities vary in different body regions, among people or over time are not yet well understood.

In a study funded in part by NIH's National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), Dr. Rob Knight and his colleagues at the University of Colorado, Boulder, and George Washington University, St. Louis, began to chart a baseline map of the human microbiota in healthy people. The results were published on November 5, 2009, in the advance online edition of Science.

The scientists surveyed bacterial communities in up to 27 different locations on the bodies of 9 healthy adults. Sampled regions included hair on the head, ear canals, nostrils, mouth, lower gut and 18 different skin sites ranging from foreheads and armpits to navels and feet. Swabs from these regions were collected 4 times over 3 months.

As in other recent studies of the human microbiota, Knight and colleagues identified bacteria by extracting DNA from each sample and then analyzing a bacteria-specific gene, called the 16S ribosomal RNA gene. Overall, the detected microbes belong to 22 bacterial phyla. Four phyla were dominant, representing more than 90% of the identified bacteria.

The researchers found wide variability in bacterial communities on each person and between people. The greatest diversity over time was seen on hair, nostril and ear canal sites, as well as some skin regions, especially the forearms, palm, index finger, back of the knee and sole of the foot. These regions were also the most divergent between people, as was the lower gut. The mouth had the least bacterial variability of any tested region.

The researchers also tested how well bacteria from one body region could survive on another. They transferred bacteria from the tongue to the disinfected forearms and foreheads of some volunteers and tracked them for up to 8 hours. Tongue bacteria persisted longer on the forearms than foreheads, suggesting that the oily forehead may be too harsh a habitat for some bacteria. Bacterial communities transplanted from forehead to forearms and vice versa could not survive well in the new habitats, coming to resemble the native mix rather than the transplants within hours.

"This is the most complete view we have yet of the microbial side of ourselves, one that our group and others will be adding to over the coming years," says Knight. "If we can better understand this variation, we may be able to begin searching for biomarkers for disease."

—by Vicki Contie

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