Further characterization of an unusual plant Mollicutes species of uncertain taxonomic status

PPAV is a type of Mollicutes that was isolated in SP4 medium from seeds of apples affected by proliferation, a disease in which mycoplasma-like organisms (MLOs) are involved. However, PPAV is probably not the etiological MLO agent for at least two reasons: 1) optimal temperature growth is 43 C, and 2) in spite of numerous isolation attempts over several years, no second PPAV culture could be obtained. PPAV is surrounded by a single cytoplasmic membrane and forms typical fried egg-shaped colonies on solid medium. The organism grows in simplified mycoplasma media, such as BSR. In growth inhibition, it shows no serological relationships with any other mycoplasmas or acholeplasmas, including those cultured from the surfaces of plants. The absence of relatedness of PPAV to other Mollicutes was confirmed by DNA hybridization studies. The genome of PPAV is close to 10(9) daltons and contains 25.2 mol % G + C. Its genome size is similar to that of Acholeplasma spp. and Spiroplasma spp. It has, however, a clearcut sterol requirement and therefore cannot be an acholeplasma. Neither is it a spiroplasma since, though filamentous in BSR medium, it has never shown signs of helicity. Hence, PPAV is a taxonomical paradox.