human microbiome: The collective genome of all microorganisms that inhabit the human body; it is now being mapped by a $115 million National Institutes of Health project.

In humans, microbial cells outnumber human cells by as many as 10 to one; some 182 species of bacteria are known to thrive on a two-centimeter-square patch of human skin. Until recently, scientists had only a glimmer of an idea how microbes affect our bodies—some help us digest food and synthesize vitamins, while others have been connected to illness, including immune diseases and digestive disorders. Beyond that general understanding, though, microbiologists haven’t made much headway because they haven’t been able to re-create the microbes’ environment outside the body to examine how they work together.

The NIH’s Human Microbiome Project, launched in December, will not only study single microbes (as in traditional microbiology) but also use a new research approach called metagenomics, which analyzes all of the genetic material derived from samples of microbial communities harvested from volunteers’ noses, mouths, skin, digestive tracts and, in women, the urogenital system. The project will begin by sampling healthy subjects to learn more about the microscopic communities that exist in a body that is functioning properly; those results will then be compared with samples from subjects with various diseases to pinpoint which combinations of microorganisms correlate to which diseases. Analysis of that data might then show how to treat patients by readjusting their microbial balance instead of prescribing drugs.