The World Within: Learning from the Microbial Inhabitants of the Human Body
Humans have always lived in a world dominated by microbes, and close relationships between microbes and humans are an ancient theme. The microbes that reside on and in the adult human body outnumber human cells by an estimated tenfold! The genomes of the microbial communities living on and within the human body (the human microbiome) contain many more genes than the human genome. Studying the human microbiome may lead to valuable new tools and guidelines in human and animal nutrition, drug discovery, and preventative medicine. Such study may also greatly expand the depth of our understanding of complex diseases including obesity, cancer, and certain immune disorders, such as asthma.
How Microbes Benefit Human Health
The vast majority of our microbial partners live in the intestine. These 10–100 trillion microorganisms perform functions such as extracting nutrients and calories from otherwise indigestible components of our diets and making essential vitamins and amino acids. Some gut bacteria also help detoxify potentially harmful chemicals contained in what we eat. Some of the microbes living in and on the human body play a critical role in defending us against pathogens. This mutually beneficial relationship helps protect us from getting diseases while giving the microbes a place to live.
Using metagenomics to gain a deeper understanding of the microbial communities in and on the human body could be immensely valuable in our understanding of both beneficial and harmful microbes and may lead to more effective ways to diagnose, treat, and possibly prevent disease.
Bacteria Are Nature's Master Chemists
The microbial world also offers a stunning array of biologically-active chemicals that bacteria have evolved through communicating with and competing against each other over billions of years. 
Bacteria are nature's master chemists; a nearly endless variety of powerful compounds can be derived from microbial communities. Hundreds of drugs available today were derived from compounds scientists originally found in microbes. Using metagenomics to study microbial communities may help us identify many more new compounds that will promote the health of humans, animals, and plants throughout the 21st century.
Case Highlight: Gut Microbes and Obesity
Metagenomics studies can help researchers learn about common diseases like obesity. Recently, metagenomics was used to compare the genes in the microbial communities in the guts of obese mice with their leaner relatives. The results suggested that obesity is associated with a shift in the proportions of the two major groups of gut bacteria. When researchers transplanted gut microbial communities harvested from both obese and lean animals into lean germ-free mice, those that received gut microbes from the obese mice gained more fat. The take home lesson: the caloric value of the food we eat may vary depending upon the composition of a person's gut microbes. Differences in our gut microbial communities may be one of many factors that affect predisposition to obesity.
The information on this Web page was derived from the National Research Council report The New Science of Metagenomics: Revealing the Secrets of Our Microbial Planet (2007) and the 20-page educational booklet derived from that report, Understanding Our Microbial Planet: The New Science of Metagenomics.
Support for this web publication was provided by the Presidents' Circle Communication Initiative of the National Academies.
