The microbes that inhabit our bodies are intimately involved in human health and disease yet we still know relatively little about them. A new major census of these tiny symbionts has revealed that they are an even more diverse bunch than was once presumed.
We have long focused on single bacteria as sources of disease (E. coli or streptococcus, for example). But we have now been learning that, for the most part, these trillions of microbes that make their homes in and on us do an excellent job keeping us healthy (crowding out harmful microbes) and sated (breaking down a lot of the food we ingest).
Now that disturbances in this rich microbiome community have been linked to weight gain, inflammatory bowel disease, vaginal infections and risk for infection with harmful microbes (such as methicillin-resistant Staphylococcus aureus, or MRSA), the importance of understanding what makes up a "healthy" microbiome has become even more apparent.
We have been adding names to the attendee list for years, but scientists still do not have a full rundown of all of the bacteria, where they live in our bodies and their role in health and disease. "We need a reference to say what is normal before we can say what is abnormal," Eric Green, director of the National Human Genome Research Institute, said in a press briefing Wednesday.
Green and his colleagues at the Human Microbiome Project have taken a big step forward in charting this complex territory, publishing an extensive new survey of the microbial profiles of hundreds of individuals. The findings are described in two papers and an essay online June 13 in Nature and in more than a dozen papers in PLoS ONE. (Scientific American is part of Nature Publishing Group.) The findings only reinforce the suspicion that this invisible landscape is even more nuanced—and important—than we thought. For example, each person might carry around hundreds of thousands of species. These bugs bring with them some eight million different genes, which far outshines our own paltry 22,000.
"This is really a new vista on biology," Phillip Tarr, director of pediatric gastroenterology and nutrition at Washington University School of Medicine in Saint Louis and a collaborator on the research, said in the press briefing. "It opens up many opportunities to improve the health of our population."
To get their results, the team collected samples from 242 healthy adults aged 18 to 40 living in Houston or Saint Louis. From each person, researchers sampled 15 to 18 specific "habitats" (nine from the mouth, four from the skin, one from the nose and three from the female genitals) as well as from stool samples.
"Healthy humans carry a remarkable diversity of organisms," says Bruce Birren, director of the Genomic Sequencing Center for Infectious Diseases at the Broad Institute and study collaborator, said at the briefing. The oral and fecal samples had the highest microbe diversity, whereas the vaginal samples had the lowest.
Each person had a relatively different microbiome, reinforcing the notion that there is no single "healthy" microbiome profile. "Apparently there are many different ways to be healthy when it comes to our microbes," Birren said. The group found that even with so many different microbial communities at each location, the same metabolic functions seem to be getting done. Birren likens it to a potluck dinner, where everyone brings something different to the table so everyone gets to eat.