Each human body harbors 10 times more microbial cells than human cells

Beginning to Understand the Effect of the Human Microbiome on Health

Each of us harbors 10 times more microbial cells than human cells in our bodies — Can we live in peace?

From shortly after we are born, each of us lives with 10 times as many microbial cells as human cells in our bodies. The metabolism of human cells influences that of microbial cells and vice versa. For that reason, the collection of microbial genes (the microbiome) has been called "the second human genome."

The microbiome has been implicated as a cofactor in diseases as diverse as obesity (JW Gen Med Jan 2 2007), inflammatory bowel disease (JW Gen Med Jun 19 2008), cancer (JW Gen Med Nov 3 2011), psoriasis, nonalcoholic fatty liver disease, insulin resistance and type 2 diabetes, asthma, and even autism.

The advent of rapid and relatively inexpensive genome sequencing (JW Gen Med Oct 4 2005) fueled international efforts to study the microbiome. The Human Microbiome Project collected repeated samples from 18 body sites of 242 healthy adults in the U.S. and reported its findings in 17 newly published reports, 2 of which are summarized here.

Along with roughly 20,000 human genes, 5 to 8 million bacterial genes exist in the microbiome. The greatest number and variety of microbes are found in the mouth and gut, and the smallest variety is found in the vagina. Different species dominate in each body niche. Microbial species are influenced by body weight, ethnic and racial background, and geographical location (e.g., developed vs. developing world).

Comment: These studies are just building a foundation for a huge new field in biology and medicine. Researchers haven't proved yet how the human microbiome affects normal human physiology or leads to disease, and some skeptics doubt they ever will. But if they do, these papers will be regarded as landmarks in the history of human biology.

— Anthony L. Komaroff, MD

Published in Journal Watch General Medicine July 24, 2012

Citation(s):

The Human Microbiome Project Consortium. Structure, function and diversity of the healthy human microbiome. Nature 2012 Jun 14; 486:207. (http://dx.doi.org/10.1038/nature11234)