Striking at the heart of what defines us as human beings, there are at least ten cells from microbes--bacteria, viruses, fungi, and other microscopic-scale denizens--for every human cell in our bodies. Providing the latest information on this remarkable human-microbe symbiosis, scientists announced new results this week from the Human Microbiome Project, a 5-year, $153 million effort funded mainly by the National Institutes of Health.
By sampling the microbiome, or population of microbes, in healthy men and women, the more than 200 researchers from 80 institutions participating in this project have already gained insights into the surprisingly fine line between health and disease in humans. The latest rounds of studies were published in this week's issue of Nature, as well as the Public Library of Science journals.
Obtaining microbial samples from 242 human subjects in up to 18 body sites such as the skin, nose, mouth, and intestines, researchers have thus far catalogued over 10,000 species of microbes. To illustrate the uncharted frontier that this endeavor represents, the researchers found small amounts of a previously unidentified type of microbe in the stools of 11 people.
Seen another way, humans contribute just 22,000 of the estimated 8 million genes that encircle and envelop the human body. The researchers sequenced the DNA of selected microbes, a possibility nowadays because of fast DNA sequencing.
The most highly reported finding from the project this week has been the fact that populations of disease-causing microbes normally exist in healthy humans. These potentially pathogenic microbes co-exist with more beneficial ones, which, for example, aid digestion in the human gut. So when disease strikes, there must be some sort of shift in the population or role of the various microbes in the body. This is perhaps analogous to how a city may always include active criminals, but a crime wave may emerge due to overall conditions in the city, such as a lack of effective law enforcement or a reduction in vigilance from citizens' groups.
Another finding of the study, mentioned in accounts such as the one in the New York Times, is that babies are "microbe magnets," building up their unique collections of microbial populations as soon as they are born. Babies in vaginal births pick up different microbes than those delivered with Caesarean sections.
Another surprising finding, as mentioned in a Washington University School of Medicine news release, is how provincial microbes can be: microbe communities on the teeth differ greatly from those in nearby saliva.
While microbe cells may greatly outnumber human cells, they are generally much smaller in size and weight. For example, the sum total of bacteria in a human body weighs only several pounds.
But once you learn how much microbe cells outnumber human cells, it's almost impossible to see ourselves the same way again. And with this recent knowledge, the microbiome may completely change our concepts of what causes human disease.