Researchers unveiled Wednesday a complete genetic panorama of microbes in the human digestive track - an advance that could help cure ailments like ulcers and inflammatory bowel disease (IBD).
"This completely changes our vision," said Stanislav-Dusko Ehrlich, a researcher at France's National Institute for Agricultural Research, after the study was published in the journal Nature.
Knowing which core bacteria populate a healthy intestine should lead to more accurate diagnosis and prognosis for diseases ranging from ulcers to IBD to Crohn's, which also causes painful inflammation, he said.
"In the future, we should be able to modify the (microbial) flora to optimise health and well being," he told AFP.
"This also opens up the possibility of prevention through diet, and treatments tailored a person's genetic and microbial profile."
More than 100 researchers working over two years found some 3.3 million distinct genes spread across at least 1,000 species of single-celled organisms, virtually all bacteria.
"The study is a blueprint," said co-author Jeroen Raes, a scientist at Vrije University in Brussels.
"The vast majority of bacteria found were not known before. But now we can start sorting out what they do in terms of function, and how they might relate to disease," he told AFP.
The intestinal census was carried out on 124 adults -- some healthy, others obese or suffering from IBD -- from Denmark and Spain.
Using new DNA sequencing techniques, scientists gathered a mass of data equivalent to 200 complete human genomes, Raes said.
The number of bacteria discovered is double many previous estimates.
But the big surprise was not the diversity, said researchers, but the fact that most humans -- despite different diets and environments -- appear to share a sizeable least common denominator of microbial flora.
Previous studies had suggested that there was relatively little overlap, especially from different corners of the globe.
Each individual in the study had at least 160 different species of micro-organisms, adding up to more than half-a-million separate genes, the researchers found.
About 40 percent of these genes were shared with at least half of the other volunteers.
There are 10 times more microbes in the body than there are human cells, with trillions of bacteria concentrated in the mouth, skin, lungs and especially the gut.
Microbes are essential to health, helping to break down indigestible foods, activate our immune system, and produce vitamins.
But recent research also points to previously unsuspected roles in obesity, heart disease and intestinal disorders such as Crohn's disease.
The new research also sets a benchmark in the methods used to sift through billions of bits of genetic code.
"This enormous sequencing effort -- the largest of its kind -- was made possible by the use of novel technologies," said Raes.
With the so-called Illumina Genome Analyser "you get huge bags of very, very small bits of DNA," he explained.
"Putting that puzzle back together again is an enormous task. Many people believed that it would not be possible."
Much of the sequencing was done by a team at the Beijing Genome Institute.