The Genus Mycoplasma and Related Genera (Class Mollicutes)
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The Genus Mycoplasma and Related Genera (Class Mollicutes) SHMUEL RAZIN
Introduction Mycoplasmas are distinguished phenotypically from other bacteria by their minute size and total lack of a cell wall. Taxonomically, the lack of cell walls is used to separate mycoplasmas from other bacteria and to place them in a class named “Mollicutes” (mollis, soft; cutis, skin, in Latin). The classification of Mollicutes and the properties distinguishing the established taxa are presented in Table 1. Though the trivial terms mycoplasmas or mollicutes have been used interchangeably to denote any species included in Mollicutes, the trivial names “ureaplasmas,” “entomoplasmas,” “mesoplasmas,” “spiroplasmas,” “acholeplasmas,” “asteroleplasmas,” and “anaeroplasmas” are routinely used for members of the corresponding genus. Molecular characterization of the uncultured plant and insect mycoplasma-like organisms (MLOs) has provided strong experimental support for their inclusion in the class Mollicutes (see the section Phylogeny and Taxonomy). Consequently, the trivial term “phytoplasmas” has been proposed to replace the awkward name “mycoplasma-like organisms.” About 200 established species have already been described within the class Mollicutes, and this number continues to rise, including the still uncultured mollicutes, such as the phytoplasmas and the hemotrophic parasites Haemobartonella and Eperythrozoon species, recently identified as mollicutes and given the trivial name “hemoplasmas” (Neimark et al. 2001). The interested reader is referred to a number of books on various aspects of mycoplasmology published during the last decade or so (Maniloff et al., 1992; Rottem and Kahane, 1993; Razin and Tully, 1995; Tully and Razin, 1996; Miles and Nicholas, 1998). The most recent multi-authored treatise covering the molecular biology and pathogenicity of mycoplasmas has been published in the summer of 2002 (Razin and Herrmann, 2002). Several recent general reviews on the molecular biology, genetics and pathogenicity of mycoplasmas are also available (Dybvig
and Voelker, 1996; Baseman and Tully, 1997; Razin et al., 1998).
Morphology The total lack of a cell wall explains many of the unique properties of the mycoplasmas, such as sensitivity to osmotic shock and detergents, resistance to penicillin, and formation of peculiar fried-egg-shaped colonies (Razin and Oliver, 1961; Fig. 1). Thin sections of mycoplasmas reveal that the cells are built essentially of three organelles: the cell membrane, ribosomes, and a circular double-stranded DNA molecule (the typical prokaryotic genome; Fig. 2). Because they are devoid of a cell wall, mycoplasmas are Gram negative. Being bounded by a plastic cell membrane only, mycoplasma cells are predominantly spherical (0.3–0.8 µm in diameter). Yet, many can be pear-shaped or flask-shaped with terminal tip structures, and form filaments of varying length (some branching) as well as helical filaments (Figs. 3–5). Maintaining such shapes
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