By Paul Rincon
BBC News Online science staff
Doctors claim to have uncovered new evidence that the tiny particles known as "nannobacteria" are indeed alive and may cause a range of human illnesses.
The evidence is suggestive of life forms, say the researchers (Image: American Physiological Society)
The existence of nannobacteria is one of the most controversial of scientific questions - some experts claim they are simply too small to be life forms.
But US scientists report they have now isolated these cell-like structures in tissue from diseased human arteries.
Their research is described in the American Journal of Physiology.
The team, led by Dr John Lieske at the Mayo Clinic in Rochester, conducted an analysis of calcified and non-calcified arteries, arterial plaques and heart valves collected as surgical waste from two US hospitals.
In the lab, they stained the specimens and examined them under a high power electron microscope.
The team found tiny spheres ranging in size from 30-100 nanometres (nm - billionths of a metre), which is smaller even than many viruses.
When the tissue was broken up, filtered to remove anything more than 200nm and the filtrate added to a sterile medium, the optical density - or cloudiness - of the medium increased.
This, the researchers argue, means the nanoparticles were multiplying of their own accord.
"I think we've taken a systematic approach to evaluating the participation of these potential nanoparticles, nannobacteria - whatever you want to call them - in human disease processes," co-author Dr Virginia Miller, also of Mayo Clinic told BBC News Online.
Spheres of influence
The particles are also recognised by a dye for DNA and absorbed uridine, a key chemical component of RNA, which the researchers argue is evidence the particles are constantly synthesising nucleic acids.
Viewed with electron microscopy, the particles also appeared to have cell walls.
The nano-scale objects showed up in tissue from patients with calcified arterial aneurysms but not uncalcified samples.
Nannobacteria have been implicated by some scientists in the formation of kidney stones and psammona bodies - calcified (mineralised) structures in ovarian cancer.
But many other scientists dispute that they are actually life forms.
"I don't see any convincing evidence for nannobacteria or DNA [in this study]," Dr John Cisar, of the National Institutes of Health in Bethesda, US, told BBC News Online.
The particles were seen in arteries with calcified aneurysms (Image: American Physiological Society)
"If you know you're dealing with a life form, you can use the staining techniques [they used]. But there are false positives in these types of techniques."
Dr Cisar said in research he had conducted, nanoparticles had tested positive with a stain for nucleic acids. But when he and his team tried to extract these nucleic acids, none had been found.
Previous research carried out by Jack Maniloff of the University of Rochester in New York has shown that to contain the DNA and proteins it needs to function, a cell must be a minimum of 140nm across.
"One of the questions we always get back is: 'well, how do you know it's alive if it doesn't have a unique DNA sequence?' This is true," Dr Miller explained.
"But if you go back to how we defined life prior to our knowing about DNA, our criteria was that things multiplied in culture. This is what we have."
In 1996, nannobacteria came to the attention of the world's media when scientists announced they had found fossils in a Martian meteorite of what appeared to be nano-sized bacteria.
Scientists are now involved in efforts to isolate DNA from the nanoparticles. Dr Miller said it was also important to investigate their role in other diseases.
The research is also reported in this week's New Scientist magazine.