A consortium of researchers organized by the National Institutes of Health have mapped the microbial makeup of healthy humans for the first time, revealing a detailed new picture of the single-cell organisms which are crucial elements of digestion, wellness, and disease.

“Enabling disease-specific studies is the whole point of the [project],” says Barbara Methé, a professor in the J. Craig Venter Institute, and lead co-author of an article in the journal Nature on the framework for current and future human microbiome research. “Now that we understand what the normal human microbiome looks like, we should be able to understand how changes in the microbiome are associated with, or even cause, illnesses.”

The Human Microbiome Project Consortium enlisted some 200 members from nearly 80 universities and scientific institutions to report on five years of research. Their work, at an NIH-paid cost of $173 million to date, has created the most extensive public reference database of the normal human microbiome by using genome-sequencing techniques to detect microbes in healthy volunteers.

The Human Microbiome Project took advantage of the ever-falling cost of sequencing DNA and a well-coordinated division of labor to undertake a large-scale survey of the microbes that inhabit just about every part of the human body. While just a few hundred bacterial species had been isolated from the body before the project was started, researchers have now cataloged between 81 percent and 99 percent of the estimated more than 10,000 microbial species that occupy the human ecosystem.

“We have defined the boundaries of normal microbial variation in humans,” says James Anderson, director of the NIH Division of Program Coordination, Planning and Strategic Initiatives. “We now have a very good idea of what is normal for a healthy Western population and are beginning to learn how changes in the microbiome correlate with physiology and disease.”

Studies to illuminate the associations of the microbiome with disease—including one on the nasal microbiome of children with unexplained fevers, and others to study the gut microbiome in Crohn’s disease, ulcerative colitis and esophageal cancer—are still in their earliest stages. But the pioneering work is likely to have a long-lasting impact on scientists’ understanding of how the microbiome supports overall health.