The Mollicutes, also called MLO or PPLO, are an unusual group of bacteria distinguished by the absence of a cell wall; a cell wall is found in most other groups. The class of Mollicutes include phytoplasmas and spiroplasma which are bacterial plant pathogens associated with insect vectors. Phytoplasma and spiroplasma are the smallest prokaryotes due to absence of the cell wall. They performed persistent propagative transmission when infection to the plant host by insect vectors. They are commonly called mycoplasmas, and are primarily parasites of various animals and plants, living within the host's cells. Individuals are very small, typically only 0.2-0.3 μm in size, and vary in form, although most have sterols that make the cell membrane somewhat more rigid. Many are able to move about through gliding. The unusual genus Spiroplasma is helical and moves by twisting.
In addition to their simplified structure, Mollicutes have simplified genomes. For instance, Mycoplasma genitalium has a total genome size of 580 000 base pairs, one of the smallest yet sequenced. Although they have been considered as possible candidates for the most primitive bacteria, it now appears they developed from the gram-positive Firmicutes, and they are now classified with them.