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. 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.

Nanobacteria are a putative novel life form first identified in the 1990s and implicated in the origin of life and in a variety of diseases. Now it seems that nanobacteria may simply be calcium carbonate nanoparticles, according to research by Jan Martel of Chang Gung University, in Taiwan, and John Ding-E Young of Rockefeller University (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0711744105). Martel and Young used published methods to "culture" nanobacteria from human blood and compared the resulting nanospheres to calcium compounds likely to precipitate in biological fluids. They found that calcium carbonate nanoparticles (shown) prepared in cell culture media from (NH4)2CO3 and CaCl2 closely resemble previously reported isolated nanobacteria. Specifically, the CaCO3 nanoparticles appear to have an amorphous layer at the surface that looks like a membrane, and two adjacent particles seem to be undergoing cell division. The researchers also found that purported nanobacteria-specific antibodies, which are commercially available, may bind to serum albumin and form an insoluble matrix with CaCO3. Before now, CaCO3 particles had only been identified in the inner ear, so finding them in other tissues such as blood may have implications for disease pathology.

Recent evidence suggests a role for nanobacteria in a growing number of human diseases, including renal stone formation, cardiovascular diseases, and cancer. This large body of research studies promotes the view that nanobacteria are not only alive but that they are associated with disease pathogenesis. However, it is still unclear whether they represent novel life forms, overlooked nanometer-size bacteria, or some other primitive self-replicating microorganisms. Here, we report that CaCO3 precipitates prepared in vitro are remarkably similar to purported nanobacteria in terms of their uniformly sized, membrane-delineated vesicular shapes, with cellular division-like formations and aggregations in the form of colonies. The gradual appearance of nanobacteria-like particles in incubated human serum as well as the changes seen with their size and shape can be influenced and explained by introducing varying levels of CO2 and NaHCO3 as well as other conditions known to influence the precipitation of CaCO3. Western blotting reveals that the monoclonal antibodies, claimed to be specific for nanobacteria, react in fact with serum albumin. Furthermore, nanobacteria-like particles obtained from human blood are able to withstand high doses of {gamma}-irradiation up to 30 kGy, and no bacterial DNA is found by performing broad-range PCR amplifications. Collectively, our results provide a more plausible abiotic explanation for the unusual properties of purported nanobacteria.