There are more than 200 known species in the class of Mollicutes (cell wall–free bacteria). At least 16 of these species are thought to be of human origin; others have been isolated from animals and plants. In humans, four species are of primary importance: Mycoplasma pneumoniae causes pneumonia and has been associated with joint and other infections. Mycoplasma hominis sometimes causes postpartum fever and has been found with other bacteria in uterine tube infections. Ureaplasma urealyticum is a cause of nongonococcal urethritis in men and is associated with lung disease in premature infants of low birth weight. Mycoplasma genitalium is closely related to M pneumoniae and has been associated with urethral and other infections. Other members of the genus Mycoplasma are pathogens of the respiratory and urogenital tracts and joints of humans and animals.
The smallest genome of mycoplasmas, M genitalium, is little more than twice the genome size of certain large viruses. Mycoplasmas are the smallest organisms that can be free living in nature and self-replicating on laboratory media. They have the following characteristics: (1) the smallest mycoplasmas are 125–250 nm in size; (2) they are highly pleomorphic because they lack a rigid cell wall and instead are bounded by a triple-layered "unit membrane" that contains a sterol (mycoplasmas require the addition of serum or cholesterol to the medium to produce sterols for growth); (3) mycoplasmas are completely resistant to penicillin because they lack the cell wall structures at which penicillin acts, but they are inhibited by tetracycline or erythromycin; (4) mycoplasmas can reproduce in cell-free media; on agar, the center of the whole colony is characteristically embedded beneath the surface; (5) growth of mycoplasmas is inhibited by specific antibody; and (6) mycoplasmas have an affinity for mammalian cell membranes.
Morphology and Identification
Mycoplasmas cannot be studied by the usual bacteriologic methods because of the small size of their colonies and the plasticity and delicacy of their individual cells. Growth in fluid media gives rise to many different forms. Growth on solid media consists principally of protoplasmic masses of indefinite shape that are easily distorted. These structures vary greatly in size, ranging from 50 to 300 nm in diameter. The morphology appears different according to the method of examination (eg, darkfield, immunofluorescence, Giemsa-stained films from solid or liquid media, and agar fixation).
Culture of mycoplasmas that cause disease in humans requires media with serum, a metabolic substrate such as glucose or urea, and growth factors such as yeast extract. There is no one medium that is optimal for all the species because of different properties and substrate requirements. After incubation at 37°C for 48–96 hours, there may be no turbidity in broth cultures; however, Giemsa stains of the centrifuged sediment show the characteristic pleomorphic structures, and subculture on appropriate solid media yields minute colonies.