Size also matters to biochemists. Conventional wisdom places the lower bound on life at about 1 micron (give or take a few hundred nanometers). In 1999, the National Academy of Sciences sponsored a workshop on the minimal size requirements for life and concluded that life can’t exist if it’s less than 200 to 300 nanometers in size. This lower limit stems from the fact that the simplest conceivable living organism requires about 250 to 400 different proteins plus the genes and ribosomes needed to make them.
Some scientists, however, have challenged the prevailing view on these minimum size limits. They claim that they have evidence for bacteria well under 100 nanometers in size, so-called nanobacteria. Maybe size isn’t as important as biochemists think. There have been basketball players under six feet tall, like Calvin Murphy who had a successful career in the NBA even though he is only 5’ 9”. In fact, Murphy was elected to the Basketball Hall of Fame.
Even though nanobacteria are diminutive, they may make a big impact in a number of scientific arenas. Some biomedical researchers have implicated these putative nano-microbes in gallbladder and kidney stone formation, rheumatoid arthritis, some forms of cancer, and even Alzheimer’s disease. Some microbiologists claim to have detected nanobacteria in geological samples and have suggested that they may play a key role in some geochemical cycles. NASA scientists claimed that nanobacteria left behind fossil remains in the Martian meteorites ALH84001, opening up the possibility that life originated on Mars.
If nanobacteria exist then what are they? There is much debate about the identity of nanobacteria. Some claim that they are organisms that push the bounds of what we currently think is possible for life. Proponents of this view point out that before the discovery of extremophiles, few would have thought that life could exist, let alone thrive, under the extreme conditions of high temperatures and pressures, high concentrations of salt, or under highly acidic or alkaline conditions. Could it be that nanobacteria challenge our current understanding about the size requirements for life?
Some think that nanobacteria represent a transitional form of sorts. They speculate that these organisms are some type of primitive self-replicating system that connects contemporary life to more primitive life-forms.
Others think that nanobacteria represent a completely novel form of life—life as we don’t know it.
New work published in the Proceedings of the National Academy of Sciences sheds light on the identity of nanobacteria and has broad-reaching implications for such considerations as pathogenesis (origin of disease), the definition of life, the origin of life, and the possibility of life on Mars.
Evidence for Nanobacteria
Support for the existence of nanobacteria comes primarily from biomedical work and includes: 1) the ability to cultivate slow-growing cultures in growth media after inoculation from human serum; 2) the ability to raise antibodies to nanobacterial antigens; 3) bacteria-like appearance of nanobacteria when visualized with electron microscopy (such as uniform size, vesicular shapes that appear to be delineated by a membrane, structures that appear to capture cell division, and aggregation into colonies); and 4) the recovery of 16srRNA gene sequences associated with nanobacteria. In fact, a biotech company Nanobac Pharmaceuticals has recently been launched to commercialize diagnostic and treatment protocols for pathogenic nanobacteria.
An Early Challenge to Nanobacteria
In the midst of all the excitement about nanobacteria, some researchers have expressed skepticism about their bio-authenticity. For example a study published in 2000 argued that nanobacteria are actually inorganic deposits of hydroxyapatite. (See here for further discussion.) The results of this work, however, have been largely ignored.
A More Recent Challenge to Nanobacteria
A new study, just published, takes up where the older work left off. This new research affirms that nanobacteia are inorganic in nature, not biological entities. The researchers demonstrated, however, that instead of being hydroxyapatite, nanobacteria are calcium carbonate precipitates. These workers were able to grow nanobacteria-like particles from calcium carbonate that had uniform size and shapes, and appeared to be bound by a membrane with some appearing to undergo a cell-division process. These precipitates also aggregated into colony-like structures. The formation of these precipitates did require the presence of human serum. Presumably the proteins in the human serum controlled the morphology of the calcium carbonate deposits. It turns out that the response of the nanobacteria-like precipitates to antibodies seems to originate from human serum protein contaminants associated with the calcium carbonate.
Other results which favor the inorganic interpretation of nanobacteria include: 1) the failure to detect any DNA associated with the nanobacteria-like material; 2) the ability to generate growth after filtering the serum-inoculated growth medium through 0.1 micron pores; and 3) the ability to generate growth after irradiation with gamma-rays.
The implications of this latest work are profound. If nanobacteria are inorganic then:
- Nanobacteria do not cause disease.
- It appears that life can’t exist below about 250 to 300 nanometers in size. This means that life requires a significant level of complexity to even exist (250 to 400 different proteins plus the genes and ribosomes needed to make them).
- Nanobacteria are not a primitive self-replicating entity.
- The evidence for ancient life on Mars is negated.
The long and the short of it (no pun intended; well, actually, it was intended), nanobacteria are not life’s version of Calvin Murphy.