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THE nooks and crannies of a healthy person are home to trillions of micro-organisms. Without this non-human component of the body, which makes up about 1 per cent to 3 per cent of a person by weight, we could not survive.

These minuscule helpers digest our food, strengthen our immune system and ward off dangerous pathogens.

They can also distinguish us, one from another.

Humans have more than 10,000 species of microbes living in and on them, but individuals can carry around very different sets of microbial populations, new research shows.

While most of these constant bedfellows are friendly, nearly everyone carries some potential foes that can cause disease under the right conditions.

These are the findings, released today, of the Human Microbiome Project – the first comprehensive census of the micro-organisms, such as bacteria, viruses and fungi, living on a healthy body.

More than 240 people between the ages of 18 and 40 donated samples from up to 18 sites on their body including the skin, mouth, nose, lower intestine and vagina, for the research, a five-year, $154 million project funded by the US National Institutes of Health. A team of 200 scientists used the latest DNA analysis techniques to study what microbes were present in those samples.

The director of the institutes, Dr Francis Collins, compared the efforts of the researchers to those of past heroes.
"Like 15th century explorers describing the outline of a new continent, the Human Microbiome Project researchers employed a new technological strategy to comprehensively define, for the first time, the normal microbial make-up of the human body," he said.

The results, published in the journals Nature and PLoS, would provide new insights into human health, said a team member, George Weinstock, of Washington University. "Knowing which microbes live in various ecological niches in healthy people allows us to better investigate what goes awry in diseases that are thought to have a microbial link, like Crohn's and obesity, and why dangerous pathogens sometimes, but not always, cause life-threatening illnesses."

While a person has about 22,000 genes, the researchers estimated that bacteria contributed hundreds of times more genes – about 8 million – that help make up for human deficits.

For example, people did not have all the enzymes required to digest food, said Lita Proctor, another team member.

"Microbes in the gut break down many of the proteins, lipids and carbohydrates in our diet into nutrients that we can then absorb."

They also produced vitamins and anti-inflammatory substances that our genes couldn't make, to help regulate the immune system, she said. The ecological niches in the body differed as much as a rainforest does from a desert, the researchers said.

The skin has a diverse collection of microbes, while the vagina has fewer species. The microbes living on teeth are different from those in saliva.

Healthy people can vary a lot, with one type of bacteria making up 90 per cent of the microbes in one person's gut, but less than 1 per cent in another's. But the different gut microbes can perform the same metabolic tasks.

Researchers have already started to use the new information, examining the microbes in the noses and blood of children with sudden, unexplained high fever, a common problem in those under three. They are often treated with antibiotics as a precaution, but the research shows the sick children had more species of viruses than healthy children, and viruses are not killed by antibiotics.

Other researchers are studying a gastric disease that threatens the life of some premature babies. By comparing the microbial communities in infants with and without the condition they hope to understand why some babies develop it.

The study found about 30 per cent of the healthy people had potentially harmful golden staph bacteria in their noses.

"The future of microbiome research is very exciting," Dr Weinstock said.