Nanobacteria are claimed to be cell walled microorganisms with a diameter well below the generally accepted lower limit (about 200 nanometres) for bacteria.

Claims of their being living organisms are controversial. If they are living, there is speculation that they may be a new form of life, rather than bacteria. [...] Speculation also that observed repuduction was nothing more than crystal formation.

Working with particles less than 0.2 micrometres in size, researchers at the Mayo Clinic found indirect evidence that the particles had self-replicated, and found that they had a cell-like appearance under an electron microscope. They also believe that the particles are producing RNA, since they absorbed one of its building blocks, uridine, in greater quantities than would be expected in the case of pure absorption (by crystals such as apatite). Using an antibody produced by the Finnish researchers, the particles were found to bind to diseased arterial tissue, and to the same sites to which a DNA stain bound. The researchers now hope to isolate RNA and DNA from the particles.