1. A genus of aerobic to facultatively anaerobic bacteria (family Mycoplasmataceae) containing gram-negative cells that do not possess a true cell wall but are bounded by a three-layered membrane; they do not revert to bacteria-containing cell walls or cell wall fragments. The minimal reproductive units of these organisms are 0.2–0.3 mcm in diameter. The cells are pleomorphic, and in liquid media appear as coccoid bodies, rings, or filaments. Colonies of most species consist of a central core, growing down into the medium, surrounded by superficial peripheral growth. They require sterol for growth. They also require enrichment with serum or ascitic fluid. These organisms are found in humans and other animals and can be pathogenic. The type species is Mycoplasma mycoides.
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