Mycoplasmas & unsheathed bacteria

A. L. Samer Faisal

In humans, four species are of primary importance:

1. Mycoplasma pneumoniae causes pneumonia and has been associated with joint and other infections.

2. Mycoplasma hominis sometimes causes postpartum fever and has been found with other bacteria in uterine tube infections.

3. Ureaplasma urealyticum is a cause of nongonococcal urethritis in men and is associated with lung disease in premature infants of low birth weight.

4. Mycoplasma genitalium is closely related to M pneumoniae and has been associated with urethral and other infections.

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.

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.

6. mycoplasmas have an affinity for mammalian cell membranes.

Morphology & IdentificationTYPICAL ORGANISMS1. Ranging from 50 to 300 nm in diameter.2. The morphology appears different according to

the method of examination (eg, darkfield, immunofluorescence, Giemsa-stained films from solid or liquid media, and agar fixation).

3. Most are pseudococcoidal, but there are notable exceptions.

CULTURE• There is no one medium that is optimal for all

the species because of different properties and substrate requirements.

• These colonies are round, with a granular surface and a dark center typically buried in the agar.

GROWTH CHARACTERISTICS

• 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 develop predominantly at cell surfaces.

• Many established animal and human cell culture lines carry mycoplasmas as contaminants; often the mycoplasmas are intracellular.

INFECTION OF HUMANS

• Mycoplasmas have been cultivated from human mucous membranes and tissues, particularly from the genital, urinary, and respiratory tracts.

• Mycoplasmas are part of the normal flora of the mouth and can be grown from normal saliva, oral mucous membranes, sputum, or tonsillar tissue.

Diagnostic Laboratory Tests• SPECIMENS: Specimens consist of

throat swabs, sputum, inflammatory exudates, and respiratory, urethral, or genital secretions.

• MICROSCOPIC EXAMINATION: Direct examination of a specimen for mycoplasmas is useless.

• CULTURES: The material is inoculated onto special solid media and incubated for 3–10 days at 37°C with 5% CO2 (under microaerophilic conditions), or into special broth and incubated aerobically. Colonies may have a "fried egg" appearance on agar.

Electron micrograph of Mycoplasma pneumoniae attached to ciliated respiratory epithelial cells in a sputum sample from a patient with culture-proved Mycoplasma pneumoniae pneumonia. The organisms (M) are seen on the luminal border attached between cilia (C).