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The smallest prokaryotic microorganisms that are able to grow on cell-free artificial media. Their genome size is also among the smallest recorded in prokaryotes, about 5 × 108 to 109 daltons. The mycoplasmas differ from almost all other prokaryotes in lacking a rigid cell wall and in their incapability to synthesize peptidoglycan, an essential component of the bacterial cell wall.
Taxonomically, the mycoplasmas are assigned to a distinct class, the Mollicutes, containing two orders, Mycoplasmatales and Acholeplasmatales. The distinction between the orders is based primarily on differences in nutritional criteria: members of the Mycoplasmatales require cholesterol or other sterols for growth whereas those of the second order do not. The term mycoplasmas is generally used as the vernacular or trivial name for all members of the class Mollicutes, irrespective of the classification in a particular genus.
The mycoplasmas are almost ubiquitous in nature. Several species are important pathogens of humans, animals and plants, while others constitute part of the normal microbial flora of, for example, the upper respiratory and lower urogenital tracts of humans. Mycoplasma pneumoniae was found to be the cause of cold agglutinin-associated primary atypical pneumonia. This disease is particularly frequent in the 5–15-year age group; it is probably endemic almost all over the world and often reaches epidemic proportions at intervals of 4 to 5 years.
Mycoplasmas are generally highly resistant to benzyl penicillin and other antibiotics which act by interfering with the biosynthesis of peptidoglycan. They are usually susceptible to antibiotics that specifically inhibit protein synthesis in prokaryotes, such as tetracyclines and chloramphenicol. Susceptibility to other antibiotics, such as erythromycin and other macrolides, is variable. See Antibiotic, Bacterial physiology and metabolism, Plant pathology, Pneumonia