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This chapter includes two genera of unique microbes that lack a cell wall but otherwise resemble bacteria. They differ from viruses by having both DNA and RNA and by the ability to grow in cell-free media. They are ubiquitous in nature as the smallest of free-living microorganisms. Numerous Mycoplasma species have been isolated from animals and humans, but Mycoplasma pneumoniae stands out as the clearest and most important human pathogen. The other species associated with human disease are summarized in Table 38–1.

TABLE 38–1Features of Pathogenic Mycoplasma and Ureaplasma


Mycoplasma and Ureaplasma are taxonomically placed in the Mollicutes, a class of prokaryotes that lack a cell wall. Although their DNA does not resemble any other prokaryote, evolutionary studies suggest they are derived from Gram-positive bacteria by reductive evolution. They are very small (diameter 0.2-0.3 μm), but highly plastic and pleomorphic appearing as coccoid bodies, filaments, and bottle-shaped forms. The cells are bounded only by a single trilaminar membrane (Figure 38–1), which, unlike bacteria, contains sterols. The sterols are not synthesized by the organism, but are acquired as essential components from the medium or tissue in which the organism is growing. Flagella and pili are lacking but surface organelles mediating attachment have been identified for some species. Lacking a cell wall, Mycoplasma and Ureaplasma stain poorly or not at all with the usual stains. Their double-stranded DNA genome is small, in part due to the lack of genes encoding a complex cell wall. Mycoplasma pneumoniae is an aerobe, but most other species are facultatively anaerobic. All grow slowly in enriched liquid culture medium and on special Mycoplasma agar to produce minute colonies only after several days of incubation. For some, the center of the colony grows into the agar and appears denser, giving the appearance of an inverted “fried egg.”

FIGURE 38–1.

Electron micrograph of Mycoplasma. Note cytoplasmic membrane ribosomes and surface amorphous material with absence of cell wall. (Courtesy of the late Dr. E. S. Boatman.)

No cell walls

Cell membrane contains sterols

Not stained well by common methods

Slow growth in specialized media



In addition to the general features of Mycoplasma, M pneumoniae has a terminal organelle which is a membrane-bound protrusion of the cytoplasm ...

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