Mycoplasmatales sometimes called PPLO( pleura-pneumonia-like organisms) are the smallest known organisms and are all parasitic. They resemble the microbes causing bovine pleuropneumonia. Many types have been isolated from soil, sewage, plants and animals. The Eaton agent , causes a common type of human pneumonia; one causes ‘witches broom’ disease in potatoes, and several cause various ‘yellows’ disease in plants.
The spherical Mycoplasma laidlawi , with a diameter of about 100nm at one stage, is thesmallest known. These elementary bodies, as they are called, grow through a intermediate stage into larger bodies about 300nm in diameter. Then follows one of two courses; each large body may undergo fission to produce intermediate stages, or may undergo sporulation to produce a number of elementary bodies.
These organisms grow, reproduce, possess lipoid-protein membranes; they have a double DNA helix and ribosomes ith RNA. Theoretical calculations suggest that the lower limit of size for living organisms is about 50nm diameter with possibly 150 large molecules. The smallest PPLOs are about twice this diameter (8 x mass) and possess about 1200 large molecules. It is possible that there may be even smaller organisms in nature.