We're one-tenth human
Rest of the body is a swarm of foreign microbes
WASHINGTON -- Scientists are beginning a large-scale effort to identify and analyze the vast
majority of cells in or on your body that aren't of human origin.
Only about 10 per cent of the trillions of cells that make up a person are truly human, researchers
say. The other 90 per cent are bacteria, viruses and other microbes swarming in your gut and on
your skin.
"We really are a superorganism," Brett Finlay, a microbiologist at the University of British
Columbia in Vancouver, said in an email. "From the moment we are born until we die, we live in a
symbiotic relationship with our microbes."
"At birth, babies emerge from a sterile environment into one that is laden with microbes," said
Laurie Comstock, a microbiologist at Harvard Medical School in Boston. "The infant's intestines
then rapidly become home to one of the densest populations of bacteria on Earth."
Most of these microbes are harmless, researchers say. Many are necessary to life and health. A
troublesome minority, however, can cause everything from teenage acne and obesity to autism
and cancer.
The National Institutes of Health has launched a $115-million, five-year project to identify, analyze
and catalog hundreds of microbial species resident in or on the human body.
Called the Human Microbiome Project, it's modelled after the Human Genome Project, which
decoded most of the human genes in the 1990s. The first 35 microbiome research grants took
effect this summer.
"The composition of the complex microbial communities inhabiting the human body has a
tremendous influence on human health and disease," said Richard Gibbs, a leading human
genome researcher at the Baylor College of Medicine in Houston. Gibbs received a grant to
sequence the genes of 400 bacterial strains by 2011.
The goal of the microbiome project, which is international in scope, is to identify which microbes
are harmful and to figure out ways to prevent or treat diseases they cause.
It's a bewildering task, because scientists estimate there are about 1,000 different species of
microbes living in the human gut and about as many more separate species on human skin.
The microbes form tiny colonies of bacteria that settle in different areas of the body. Jeffrey
Gordon, a microbiologist at the University of Washington in St. Louis, likened them to
"ecosystems," similar to those that plants and animals form on islands on Earth.
The most popular site for human skin microbes, surprisingly, is the forearm, which is home to 44
different microbial species, according to a recent study by Julia Segre, a microbiologist at the
National Human Genome Research Institute in Bethesda, Md. The most barren region is behind
the ear, where only 15 species typically settle, she reported in the journal Science last May.
"Hairy moist underarms lie a short distance from dry forearms, but these two niches are as
ecologically dissimilar as rainforests are to deserts," Segre said.
Different tribes of microbes are associated with different maladies. For example, bacteria
associated with the skin disease psoriasis favour the outer elbow, Segre reported. Eczema bugs
prefer the inner elbow.
Microbes also vary between people. Matthias Tschoep, an obesity expert at the University of
Cincinnati's College of Medicine, identified 383 microbial genes that differed significantly
between pairs of obese and slender twins. Microbes in obese people harvest sugars and fats in
the diet more efficiently than do others on slender people, he reported in Nature.
"It is possible that drug targets or drug candidates for the treatment of obesity could be identified
from the obesity-associated microbiome," Tschoep said.
At the Conference on the Beneficial Effects of Microbes held in San Diego last fall, scientists
described many ways in which microbes can be helpful -- even essential -- to humans.
Bacteria in the gut make it possible to digest food, synthesize vitamins, remove toxins and
develop the immune system after birth.
One of the new human microbiome grants went to Robert Modlin, a dermatologist at the University
of California, Los Angeles, to study microbes lurking under the skin that cause 17 million
Americans -- including 80 per cent of those age 12 to 24 -- to suffer from acne.
"Success may lead to the development of new, effective therapeutic strategies for treatment of
acne," Modlin's grant announcement declared.