ABSTRACT
Mycoplasma spp. are members of the class Mollicutes, have the smallest cells among free-living eubacteria, and have genomes presumed to approach the minimal essential information for independent cellular life. Mycoplasma and the other mollicutes probably evolved from Gram-positive ancestors (Maniloff, 1992, 2002) by reductive processes that resulted in essentially obligate commensalism or parasitism of eukaryotic host cells. The distinguishing characteristics of mollicutes include small cell size (200 to 500 nm), small genome size (580 to 2200 kbp), low G+C content (typically in the range of 23 to 34 mol%, but 40 mol% in Mycoplasma pneumoniae), 16S rDNA sequences clearly afliated with the class (Weisburg et al., 1989), unique codon usage (e.g., UGA as a tryptophan codon) in some lineages, minimal metabolic capabilities, lack of any cell wall, and species-specic serology. Although their cells are bounded only by a unit membrane, resulting in a general cellular pleomorphism, cytoskeletal elements confer helicity or polarity in
Introduction .................................................................................................................................... 467 The Species Concept for Mycoplasma and Related Organisms ....................................................468 Description of the Class Mollicutes ...............................................................................................469
