MYCOPLASMAS
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 urogenital 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 known 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.
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, dark field, 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.
After 2–6 days on biphasic (broth over agar) and agar medium incubated in a Petri dish that has been sealed to prevent evaporation, isolated colonies of the more rapidly growing mycoplasmas measuring 20–500 μm can be detected with a hand lens. These colonies are round, with a granular surface and a dark center typically buried in the agar. They can be subcultured by cutting out a small square of agar containing one or more colonies and streaking this material on a fresh plate or dropping it into liquid medium.
Mycoplasmas are unique in microbiology because of (1) their lack of a call wall; (2) their extremely small size; and (3) their growth on complex but cell-free media.
Mycoplasmas pass through filters with 450-nm pore size and thus are comparable to chlamydiae or large viruses. However, parasitic mycoplasmas grow on cell-free media that contain lipoprotein and sterol. This sterol requirement for growth and membrane synthesis is unique.
Many mycoplasmas use glucose as a source of energy; ureaplasmas require urea.
Some human mycoplasmas produce peroxides and hemolyze red blood cells. In cell cultures and in vivo, mycoplasmas are observed predominantly at cell surfaces. Many established animal and human cell culture lines carry mycoplasmas as contaminants; often the mycoplasmas are intracellular as well.
The extreme pleomorphism of mycoplasmas is one of their principal characteristics.
At least 16 antigenically distinct species can be identified from humans, including M hominis, M pneumoniae, M genitalium, and U urealyticum. Most Mycoplasma species have high evolved systems for variation of outer membrane antigens presumably for evading the host immune response during infection.
The species are classified by biochemical and serologic features. The complement fixation (CF) antigens of mycoplasmas are glycolipids. Antigens for enzyme-linked immunoassay (ELISA) tests are proteins. Some species have more than one serotype.
Many pathogenic mycoplasmas have flasklike or filamentous shapes and have specialized polar tip structures that mediate adherence to host cells. These structures are a complex group of interactive proteins, adhesins (eg, the P1 adhesin of M pneumoniae and the MgPa adhesin of M genitalium), and adherence-accessory proteins. The proteins are proline rich, which influence the protein folding and binding and are important in the adherence to cells. The mycoplasmas attach to the surfaces of ciliated and nonciliated cells, probably through the mucosal cell sialoglycoconjugates and sulfated glycolipids. Some mycoplasmas lack the distinct tip structures but use adhesin proteins or have alternative mechanisms to adhere to host cells. The subsequent events in infection are less well understood but may include several factors as follows: direct cytotoxicity through generation of hydrogen peroxide and superoxide radicals, cytolysis mediated by antigen–antibody reactions or by chemotaxis and action of mononuclear cells, and competition for and depletion of nutrients.
Mycoplasmas have been cultivated from human mucous membranes and tissues, particularly from the genital, urinary, and respiratory tracts. Mycoplasmas are part of the normal microbiota of the mouth and can be grown from normal saliva, oral mucous membranes, sputum, or tonsillar tissue. M hominis is found in the oropharynx of fewer than 5% of adults. M pneumoniae in the oropharynx is generally associated with disease (see below).
Some mycoplasmas are inhabitants of the genitourinary tract, particularly in women. In both men and women, genital carriage of mycoplasmas is directly related to the number of lifetime sex partners. M hominis can be cultured from 1–5% of asymptomatic men and 30–70% of asymptomatic women; the rates increase to 20% and more than 90% positive for men and women, respectively, in sexually transmitted disease clinics. U urealyticum is found in the genital tracts of 5–20% of sexually active men and 40–80% of sexually active women. Approximately 10% of women attending sexually transmitted disease clinics have M genitalium in their lower genital tracts. The presence of M genitalium in the male urethra is typically associated with disease, a syndrome termed nongonococcal urethritis. Other mycoplasmas also occur in the lower genital tract.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
