Mollicutes

Mycoplasmatales
Entoplasmatales
Anaeroplasmatales
Acholeplasmatales The Mollicutes are an unusual group of bacteria distinguished by the absence of a cell wall, as is found in most other groups. 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.

A few Mollicutes cause diseases in humans, entering cells in the respiratory of urogenital tracts. These include certain species of Mycoplasma, Ureaplasma, and Erysipelothrix.