False-colour transmission electron micrograph (TEM) of Mycoplasma sp.. Mycoplasmas are the smallest cellular organisms known (diameter 0.3- 0.9 microns). Similar to bacteria in some respects (the relationship is unclear - some authorities regard them as primitive bacteria), they differ in that they lack a true cell wall & hence display a variety of forms. Some species are pathogens of humans and animals (cattle, sheep & goats) living on the moist mucosal membranes of the respiratory tract. Mycoplasma pneumoniae, for example, causes a pneumonia-like disease in humans. Magnification: x5000 at 6x7cm size.