Gene experts after studying the DNA of hundreds of different kinds of bacteria in the human gut have reached the conclusion that the human race may not be quite as human as we may think.

Scientists from the Institute for Genomic Research (TIGR) in Maryland say that because bacteria are so important to key functions such as digestion and the immune system that we may be in fact be symbiotic organisms - relying on one another for life itself.

The team believe their findings suggest that studying bacteria native to our bodies may provide important clues to disease, nutrition, obesity and how well drugs will work in individuals.

Scientists have known for some time that at least 50 percent of human faeces, and often more, is made up of bacteria from the gut.

Bacteria start to colonise the intestines and colon shortly after birth, and adults carry up to 100 trillion microbes, representing more than 1,000 different species.

The researchers say the body is a mixture of bacteria and human cells and some scientists believe as many as 90 percent of the cells on our body are actually bacteria.

This microbial population is what dictates our well-being and any change or shift within this population which may lead to the absence or presence of beneficial microbes, can trigger defects in metabolism and the development of diseases such as inflammatory bowel disease.

The microbial elements help humans to digest much of what they eat, including some vitamins, sugars, and fibre and also synthesise vitamins that humans cannot.

The researchers say humans have evolved for million of years with these bacteria and they provide essential functions.

Microbiologist Steven R. Gill and his team sequenced the DNA in faeces donated by two healthy adults who had not taken antibiotics for a year, as these drugs are known to disturb the bacteria in the body.

The duo comprised one male and one female; one was a vegetarian.

By comparing the gene sequences to those from known bacteria and to the human genome they found the so-called colon microbiome, the entire sum of genetic material from microbes in the lower gut which includes more than 60,000 genes, which is twice as many as found in the human genome.

Gill says of all the DNA sequences in the material, only one to five percent of it was not bacterial.

They also found a surprising number of Archaea, also known as archaebacteria, which are genetically distinct from bacteria but which are also one-celled organisms often found in extreme environments such as hot springs.

Gill and his team now plan to make a comparison of the gut bacteria in people from different ethnic backgrounds, different diets, drinkers and smokers in order to see if there are distinct differences.

They believe the bacteria help break down the drugs that people take and studying the effects of different populations of the microbes might provide clues to treating different people with various medications.

Gill says by sampling the gut microbiome periodically, as well as those of other sites, such as the mouth and skin, scientists may be able to determine the effects of environmental change on our microevolution.

Gill says the next study will focus on the bacteria in the mouth, of which there are at least 800 species.

The research is published in the current issue of the journal Science.