Called a big feat in microbial research, researchers have decoded all the bacterial genes found in the human gut-a total of 160 species.
The discovery has far-reaching implications for human health and well being, and could even be used to predict chronic intestinal illnesses, from ulcers to cancer, reports the Independent.
The study decoded the DNA sequence of the many thousands of genes used by the vast number of bacteria, which take up permanent residence inside the human intestines.
It found that about 1,000 different species of microbe can live in the healthy human gut and that each person on average has about 160 species living inside them at any one time - and most of these species are common to different people.
Studying gut bacteria has been difficult, because many are unable to be grown outside of their natural habitat, and that is why the scientists believe their findings will shed light on a little-understood or discussed aspect of human biology.
It is estimated that a healthy human gut contains about 100 trillion microbial cells, about 10 times as many cells as there are in the human body.
Yet not much is known about what these bacteria do to maintain health and well-being, said Jeroen Raes of Vrije University in Brussels.
"We have no clue as to how the gut works because this is a very complex ecosystem. We really don't know how that ecosystem works even though it is crucial for our well-being. We don't know how food is digested and which species do what," Raes said.
"We've basically sequenced all of their genomes at once. It was a huge effort because it's basically the biggest sequencing exercise anyone has done so far - it's about 200 times the sequencing effort of the human genome project," he added.
The scientists took fecal samples from 124 Europeans and analyzed the DNA they contained, using powerful "gene machines" that could quickly decipher the order of the genetic "letters" running along the length of each DNA molecule, the unit of inheritance.
They used a technique called metagenomics, which attempts to sequence every scrap of DNA in a scrambled sample without first having to isolate each and every microbial species.
With these sequences it is possible to work backwards to estimate how many microbial species are present, said Jun Wang of BGI-Shenzhen in China, one of the world's biggest genome research centers.
"From all the genes in the human gut, over 99 per cent of them are bacterial, indicating that the entire cohort harbors between 1,000 and 1,150 prevalent bacterial species and each individual person has at least 160 such species, which are largely shared [from one person to another]," said Wang.
"Our intestine is home to our largest collections of microbes. Bacterial densities in the colon [large intestine] are the highest recorded for any known ecosystem... the surprise has been the gut microbes correlated so well with human health. We have to really study the 'other genome' of ourselves," he added.
Raes said the study was a technical tour-de-force because it involved the simultaneous mass screening of so many different kinds of microbes, some of which are new to science and have never before been studied.
"We've used this novel DNA sequencing technology to build a big map of all of the genes of the bacterial flora in our gut. We found about a 1,000 species of bacteria and we hardly know who they are and we definitely don't know what they are doing," said Raes.
"It was very surprising for us to find that we have so much more in common than we thought we had. The guts of different individuals have a substantial overlap in terms of species composition and function because it was always thought that human gut flora was very variable," he added.
The study has been published in the journal Nature.