Finnish researchers Olavi Kajander and Neva Ciftcioglu says the bacteria - known as nanobacteria - float around in the blood and its fluid, serum, and are also present in kidney stones.

They believe the bacteria build themselves small mineral shell homes out of a substance called apatite - which is also found in bones and teeth.

Writing in the Proceedings of the National Academy of Scientists, the researchers say: "Nanobacteria are the smallest cell-walled bacteria, only recently discovered in human and cow blood and commercial cell culture serum.

"The apatite shelters...were apparently the dwelling place of the organisms."

Layer by layer build-up

They believe the nanobacteria and their homes could eventually build up layer by layer and form kidney stones - just as a grain of sand in an oyster can form a pearl.

Mars: linked to kidney stones?

The researchers looked at the make-up of 30 kidney stones and found the nanobacteria in all of them.

They believe blood serum contains chemicals which stops the build-up of nanobacteria, but that they can be transported to the kidneys by the blood where they may build up.

"Apatite may play a key role in the formation of all kidney stones," they said. They are looking into whether nanobacteria could be behind other diseases, such as gallstones.

Som scientists believe there may have been similar nanobacteria on Mars, but there are conflicting views. Those backing the nanobacteria theory say fossils have been found in meteorites on the surface of Mars which suggests their presence.

The fossils suggest organisms of the same shape and size as the nanobacteria found in human blood.