'Rainforest' of bacteria lives in our belly buttons: study

US researchers found 2,368 species of bacteria, 1,458 of which may be new to science, after two years of literal navel gazing.

Just eight of these species were found in more than 70 per cent of individuals sampled. However, questions remain as to what factors govern which species will be found on which people, the Daily Mail reported.

"The common, abundant species are from a relatively small number of evolutionary lines, indicating that they have evolved traits that make them at home on human skin," said Rob Dunn of North Carolina State University.

"However, we are still trying to figure out what determines which of these species are found in a given person's belly button," Dunn said.

"We've looked at sex, age, ethnicity and a number of other factors