Published online 17 April 2008 | Nature | doi:10.1038/news.2008.762

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Tiny 'living' particles may just be lumps of limestone.

They look like tiny bacteria, have been implicated in several diseases and have even been hailed as a completely overlooked branch of the tree of life. But are 'nanobacteria' genuinely alive? New research suggests that the answer is probably no.

Ever since they were first described in the early 1980s, nanobacteria — which can be just 50 nanometres, or millionths of a millimetre, across — have captured the imagination of everyone from health experts to space biologists. A panel convened in 1998 by the US National Academy of Sciences concluded that the particles are too small to be alive, but that didn't stop people from being fascinated by them, and some companies even say that they can detect nanobacterial infections.

Some scientists have argued nanobacteria could be the source of life on Earth. J. Martel/J. Young

But now the nanobacteria theory has taken a blow. New research suggests that, besides being too small to be alive, they may also be made of something not much more complex than simple chalk.

Death knell?

Jan Martel of Taiwan's Chang Gung University and John Young of Rockefeller University in New York created particles that look exactly like nanobacteria by incubating human serum with the chemical ingredients of simple calcium carbonate, or limestone.

The researchers then compared their creations with naturally occurring 'nanobacteria-like particles' from human blood samples. The particles not only looked identical to the limestone mix, they also showed no traces of DNA or RNA. The researchers then blasted the particles with enough radiation to slaughter any bacteria, and found that the particles still looked the same.

"I am pretty confident this will put an end to the biotic mechanism for nanobacteria," says Young, who reported the research in Proceedings of the National Academy of Sciences1.

But why were some biologists so eager to believe that these were genuine life forms? Because the particles grow in such a life-like way, says Young. They even split in two in a way that resembles dividing bacteria. "They seem to grow, to propagate, and divide — you would swear they are a biological organism — but they're not," he says.

What is most intriguing about nanobacteria is that, although they seem to form by a chemical reaction between calcium and carbon dioxide — both of which are found naturally dissolved in blood — they are not simple lumps of chalk. "We believe what we have found is a complex of organic molecules and minerals," Young says.

That means that nanobacteria probably become coated with proteins or other organic molecules, which stop them from growing like a regular, jagged limestone crystal and instead look like round blobs.

It is these organic coatings that might have tricked experts into thinking that nanobacteria are alive, suggests John Cisar, a microbiologist at the US National Institutes of Health in Bethesda, Maryland. "But the simpler explanation is that they are not life forms," he says.

Commercial interest

Still, biological tests are commercially available for nanobacteria. One company, Nanobac Pharmaceuticals of Tampa, Florida, offers immunological tests to detect antibodies against nanobacteria, or 'calcifying nanoparticles', as it calls them, in human serum or plasma.

But when Martel and Young tested this product as part of their study, they found that the antibodies detected by the test also react with albumin, a protein widely found in blood serum. Young therefore suggests that many nanobacteria in the blood may be coated in albumin, and the test detected these.

Brady Millican, a representative of Nanobac, says "there are lots of rogue proteins associated with the structure itself". He says the test kit marketed by the company, which contains two different antibodies, is an assay that "captures more of the surface of the calcifying nanoparticle".

Young does believe, however, that nanobacteria can cause disease. Given the right chemical conditions, they "aggregate like crazy", he says. Perhaps this is a cause of extra-skeletal calcification, a painful and debilitating condition in which a kind of 'bone' forms in body areas such as tendons.

Cisar, on the other hand, thinks that although calcium carbonate deposits in the body can cause some ailments, nanobacteria are probably benign because they are so widespread in the body. "Every place you look for these things you find them," he says. "There's no particular pathology."

This article is part of Nature's 'Next Week's News' experimental feature, in which readers pick the science topic they'd like to see explored in a full-length story.