Our bodies are made up of more microbes than human cells, according to a study carried out by more than 200 researchers from 80 universities.

The finding is part of a five-year study headed up by the Human Microbiome Project. It revealed that, of the 100 trillion cells that make up a human body, around one in ten are actually microorganisms such as bacteria and viruses. The results are being released in a series of reports in the journals Nature, Genome Biology and PLoS. "The human we see in the mirror is made up of more microbes than human," summed-up the Human Microbiome Project leader Lita Proctor. A rather worrying statement, one might think, however these microbes play an important role in supporting and maintaining vital functions such as our immune and digestive systems. We can't process our food without protein-, lipid- and carbohydrate-breaking microbes present, and other microbes produce necessary vitamins and anti-inflammatories.

One startling discovery in the research showed that the microbes provide more genes for human survival than humans cells, with the human microbiome contributing eight million unique protein-coding genes, versus the human genome's 22,000.


So on the flip side, you could say it takes ten microbes to sustain every human cell in the human body. Just to dispel any unappealing mental images you might have of humans as giant walking bacteria, microbes only make up between one and three percent of the body's total mass.

As part of the five-year trial, 242 healthy individuals had their microbial make-up mapped out using genome sequencing. Tissue samples from 15 sites on the bodies of male candidates and 18 on female candidates were studied to create the comprehensive outlook of the human microbiome.

It was found that most candidates carried pathogens, but these did not actually cause harm -- in a healthy candidate, pathogens make up part of a well-rounded community of microorganisms. Understanding how this balance works is key to uncovering why, in some individuals, pathogens turn rogue and cause disease. There is a line of thought that says subtle shifts in our internal environment can disrupt the microbes, causing them to relocate to parts of the body that they shouldn't. "There can be a disturbance in the immune system," explained Proctor. "There can become some kind of imbalance. And then you can get a microorganism which, under normal circumstances, lives in a benign way and can then become a disease-bearing organism."


The huge project and thorough mapping out of our internal environment should lay the groundwork for future "disease-specific studies", their causes and possible prevention. "Now that we understand what the normal human microbiome looks like, we should be able to understand how changes in the microbiome are associated with, or even cause, illnesses," says Barbara Methé of the J Craig Venter Institute involved in one of the studies.

Studies have already been launched into the role of the microbiome in Crohn's disease, ulcerative colitis, psoriasis and a series of conditions common among children.

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