"What I found most surprising was the great diversity of bacteria living on the skin," said Julia Segre of the US National Human Genome Research Institute in Bethesda, Maryland, who led the research.

According to the first big census of microbes, parts of the body such as the moist armpits were akin to tropical rainforests in terms of the type of ecosystem the bacteria inhabit, whilst other areas of skin were like dry deserts.

"The second most surprising finding was that the skin was like a desert with moist areas like streams such as the armpits, and isolated oases of life where there are rich reservoirs of deep diversity, such as the navel," said genetics specialist Segre, whose study is published in the journal Science.

The human bodies are ecosystems, believed home to trillions of bacteria, fungi and other microbes that naturally coexist in the skin, the digestive tract and other spots.

Sure they cause odour, "but they also keep your skin moist and make sure if you get a wound that (dangerous) bacteria do not enter your bloodstream," she underlined.

The landmark study found there are 100 times more bacterial species living on the skin than estimated. It discovered that the same regions of skin on different people tended to house similar communities of bacteria and that these variations in bacterial habitats may explain why some skin complaints tend to affect certain areas of the body.

Although the greatest overall number of bacteria were often found in the moist, hairy parts of the body like the armpits, they found the greatest diversity of microbes on the dry, smooth forearm, where on average they found 44 species of bacteria. The least diverse habitat was behind the ear, they said.

"Hairy, moist underarms lie a short distance from smooth, dry forearms, but these two niches are likely as ecologically dissimilar as rainforests are to deserts," the scientists said.

Dr Segre said that the study of bacterial communities on the skin will help the understanding of certain skin conditions, and help to unravel why psoriasis tends to affect the outer elbow whilst eczema often affects the inner bend of the elbow.