Any of the bacteria that make up the genus Mycoplasma .
They are among the smallest of bacterial organisms. The cell varies from a spherical or pear shape to that of a slender branched filament. Mycoplasma species are gram-negative (see gram stain ) and do not require oxygen. They are colonial microorganisms that lack cell walls. They are parasites of joints and the mucous membranes lining the respiratory, genital, or digestive tracts of cud-chewing animals, carnivores, rodents, and humans. Toxic by-products excreted by the bacteria accumulate in the host's tissues, causing damage. One species causes a widespread but rarely fatal pneumonia in humans.