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Overview
Mycoplasma are tiny bacteria that lack a cell wall. Their outer cell membrane contains sterols that they obtain from the tissues or special media in which they grow. Mycoplasma pneumoniae is second only to the pneumococcus as a cause of community-acquired pneumonia. However, M pneumoniae has a predilection for younger persons and spreads person-to-person in families or closed groups, whereas the elderly are at greatest risk for pneumococcal pneumonia. Mycoplasmal infection presents as tracheobronchitis or pneumonia with headache and a persistent nonproductive cough, often worse at night. Chest radiographs usually show unilateral patchy infiltrates without lobar consolidation, hence the term “atypical” or “walking” pneumonia. The course is almost always benign, but improvement is accelerated by treatment with doxycycline or azithromycin. In the past, diagnosis was confirmed if at all by serology or rarely by culture. Multiplex PCR platforms for respiratory pathogens are being used more frequently are changing the approach to diagnosis.
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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 M pneumoniae stands out as the clearest and most important human pathogen. The other species associated with human disease are summarized in Table 38–1.
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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), highly pleomorphic, and appear 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 ...