The most extensive catalogue of the trillions of microbes that live in and on humans - called the human microbiome - has been published by an international team of scientists.
The work by the Human Microbiome Project (HMP) Consortium establishes a benchmark for a healthy microbiome - the total number of microbes and their genetic make-up. They analysed samples from 242 people using advanced versions of the DNA sequencing machines used in the Human Genome Project, and found that bacterial cells outnumber human ones by ten to one.
Combined with information from the Human Genome Project, researchers hope to better understand how these organisms interact with their human hosts and investigate what goes wrong in diseases like inflammatory bowel disease, obesity and asthma.
'The really important impact will be everything we haven't done yet, [...] using this as a definition of what's healthy', Dr Curtis Huttenhower from the Harvard School of Public Health told Bloomberg.
The large number of microbes present emphasises their importance to even the most fundamental human tasks, such as digesting food
'Humans don't have all the enzymes we need to digest our own diet', said Dr Lita Proctor. 'Microbes in the gut breakdown much of the proteins, lipids and carbohydrates in our diet into nutrients that we can then absorb'.
The team found large variations in the number and type of microbes found between sites on the body (samples were taken from skin, nose, mouth, intestines and vagina) and from person to person. But, despite this, different combinations of microbes seem capable of doing the same jobs.
'I might have a different organism on my tongue than you do on your tongue, but collectively they bring the same genes to the party - so they are able to perform some of the same functions', Dr Bruce Birren, co-director of the Genome Sequencing and Analysis Program at the Broad Institute of MIT and Harvard, told the BBC.
Scientists are hopeful these studies will lead to new therapies and better ways of tackling infectious organisms like MRSA (methicillin-resistant Staphylococcus aureus), but Dr Michael Fischbach, a biologist at the University of California, San Francisco, who wasn't involved in the project, urged caution.
'It's going to take a long time', he told the Wall Street Journal, adding that this isn't only due to the differences between individuals, but also because we need to understand how the microbiome interacts with our own genes.
'It is still an unknown land', agrees Professor David Relman of Standford University, who wrote an introduction to the HMP publications in Nature. 'Even though it is on home turf we are still discovering new life forms on it'.
The Human Microbiome Project was launched at the end of 2007 by the US National Institutes of Health. The project received funding of $173 million and involved over 200 researchers from 80 institutions. The latest papers have been published in the journals Nature, PLoS and Genome Biology.