Ever wonder why some people receive more mosquito bites than others? The answer, according to researcher Rob Knight, Ph.D., from the University of California at San Diego (UCSD) is that the microbes living on their skin produce chemicals that make them more attractive to mosquitoes. While these tiny organisms are busy unfurling an all-you-can-eat sign on the body’s surface, other members of this microscopic community are helping control body weight, promoting a robust immune system, and contributing to the overall health of the host — you.
With studies like the National Institutes of Health Common Fund Human Microbiome Project (HMP), science is just beginning to understand how microbes in the mouth, urogenital tract, gut, and skin — known collectively as the microbiome — participate in human health. During his Jan. 10 talk for the NIEHS Distinguished Lecture Seminar Series, Knight said he hoped that one day physicians would be able to glean important health information about an individual by looking at his or her microbial profile.
The human brain weighs approximately 3 pounds, Knight said, which is about the same weight as the total number of microbes that inhabit the human body. But the microbes have more functions, more genes, and arguably more connections and complexity.
"We have about 20 thousand human genes, but there are 2-20 million microbial genes, performing in all sorts of metabolic pathways," he said.
According to Knight, roughly 400 scientists around the country are involved in the HMP. In one of the projects, called the Healthy Cohort Study, they collected bacteria from 18 different sites on the body at 3 separate time points from 250 volunteers. The process yielded four and a half trillion bases of DNA, and allowed the researchers to determine where different organisms lived on the body.
Taking the research a step further, Knight developed sequence software that measured the evolutionary distance between the microbes. Each point on the map represents all the complexity of a particular microbial community projected down to a single point (see graphic). Two points that are close together have a similar evolutionary history, whereas points further apart are more dissimilar.
Knight said the fascinating part about the map is its correlation with the Earth Microbiome Project, a crowdsourcing venture to gather tens of thousands of microbial samples from around the world. Ecologically, the microbes in the mouth are as far away, or distinct, from microbes in the gut, as microbes on a coral reef are from those on a prairie.
"It means that a few feet on your body is like thousands of miles of the earth’s surface," Knight said. "So, you don’t have one microbiome, you have a whole archipelago of microbiomes."
Effects on health
Research has shown that babies born via Caesarian section have higher rates of asthma, allergies, and in some studies, a two-fold higher risk of autism, compared with babies that pass through the birth canal. But the list of diseases does not end there. Autoimmune diseases like rheumatoid arthritis, type 2 diabetes, colon cancer, and Parkinson’s disease have also been linked to the microbiome. Furthermore, in mouse studies, the microbiome has been connected to depression and autism.
Even the way a person processes toxins or medications depends on their microbiome. Knight maintained that doing a drug trial in one population and marketing it in another population may lead scientists to derive incorrect conclusions about the drug’s efficacy and safety.
"It is no surprise that environmental health researchers have become so excited about this area," said Lisa Chadwick, Ph.D., a member of the NIEHS Extramural Research and Training Division. She handles a portfolio of research grants that focus on how the microbiome interacts with environmental chemicals.
All of the previous research led Knight and scientist Jeff Leach to co-found American Gut, the largest open-source microbiome project in the world. The project allows members of the general public to learn about the populations of microbes that share their bodies and provides researchers with a large data set to compare human microbial diversity globally.