Mycoplasma. Illustration of a group of Mycoplasma sp.. Mycoplasmas are the smallest cellular organisms known (diameter 0.3-0.9 microns). They resemble bacteria in some respects (some authorities regard them as primitive bacteria), but they differ in that they lack a true cell wall and hence display a variety of forms. They may be spherical, branched or unbranched filaments, as seen here. Some species are pathogens of humans and animals (cattle, sheep) living on the moist mucosal membranes. In humans Mycoplasma hominis may infect the genital tract; Mycoplasma pneumoniae causes a pneumonia-like disease of the lungs.