Scientists have fingered tiny bacteria as possible culprits behind kidney stones and abnormal calcium deposits in other tissues. The bacteria, described in tomorrow's Proceedings of the National Academy of Sciences, are among the smallest ever found, barely bigger than some viruses.
Biochemist Olavi Kajander of the University of Kuopio in Finland first noticed the bacteria more than 10 years ago as a white film in his mammalian cell cultures. From the film, he was able to culture the slow-growing bugs, which he dubbed nanobacteria. At 200 to 500 nanometers wide, they are one-tenth the diameter of a typical Escherichia coli. So far Kajander and his colleagues have found the nanobacteria in cattle blood, in 80% of samples of commercial cow serum in which mammalian cells are grown in the lab, and in the blood of nearly 6% of more than 1000 Finnish adults tested.
Besides their size, the bacteria have another unusual property: Under certain growing conditions, they build calcium-rich spherical shells around themselves. Now Kajander and clinical microbiologist Neva Çiftçioglu report that the structures are made of apatite, a primary component of kidney stones and other calcified deposits in tissue, but different from the calcium compound in teeth and bones.
Blood contains several proteins that inhibit apatite crystals, but Kajander speculates that the bacteria might be free to form shells if they leave the bloodstream and take up residence in tissues. The small spheres, he says, may be seeds for larger calcium deposits, such as kidney stones or the abnormal calcifications found in patients with scleroderma or some cancers. Indeed, the team was able to culture nanobacteria from all 30 human kidney stones they examined.
The find is "one of the most intriguing and fascinating additions to this area of research that I can imagine," says nephrologist and kidney-stone specialist Fredric Coe of the University of Chicago. Although nanobacteria may not cause kidney stone disease, Coe notes additional circumstantial evidence: At least four teams have reported tiny spherical deposits in the calcified plaques that often appear in the kidneys of patients who suffer from kidney stones. "I don't know that it's their bacteria," he says, "but it sure looks suspicious."