The de novo synthesized polar lipids of Mycoplasma species are rather simple, comprising primarily of the acidic glycerophospholipids PG and CL. In addition, when grown in a medium containing serum, significant amounts of PC and SPM are incorporated into the mycoplasma cell membrane although these lipids are very uncommon in wall-covered bacteria. The exogenous lipids are either incorporated unchanged or the PC incorporated is modified by a deacylation-acylation enzymatic cycle to form disaturated PC. Although their small genome, in some Mycoplasma species, other genes involved in lipid biosynthesis were detected, resulting in the synthesis of a variety of glycolipis, phosphoglycolipids and ether lipids. We suggest that analyses and comparisons of mycoplasma polar lipids may serve as a novel and useful tool for classification. Nonetheless, to evaluate the importance of polar lipids in mycoplasma, further systematic and extensive studies on more Mycoplasma species are needed. While studies are needed to elucidate the role of lipids in the mechanisms governing the interaction of mycoplasmas with host eukaryotic cells, the finding that a terminal phosphocholine containing glycolipids of M. fermentans serves both as a major immune determinants and as a trigger of the inflammatory responses, and the findings that the fusogenicity of M. fermentans with host cells is markedly stimulated by lyso-ether lipids, are important steps toward understanding the molecular mechanisms of M. fermentans pathogenicity. 1. Introduction Mycoplasmas (class Mollicutes) are the smallest and simplest self-replicating bacteria [1]. These microorganisms lack a rigid cell wall and are bound by a single membrane, the plasma membrane. Wall-less bacteria were first described 100 years ago, and now over 210 species, widely distributed among humans, animals, insects, and plants, are known. The lack of a cell wall is used to distinguish these microorganisms from ordinary bacteria and to include them in a separate class named Mollicutes. Phylogenetically, the Mollicutes are related to Gram-positive bacteria from which they developed by genome reduction [2]. Therefore, the Mollicutes are not at the root of the phylogenetic tree but are most probably late evolutionary products [3]. Most human and animal mycoplasmas are Mycoplasma species of the family Mycoplasmataceae. Because mycoplasmas have an extremely small genome (0.58–2.20？Mb compared with the 4.64？Mb of Escherichia coli), these organisms have limited metabolic options for replication and survival and depend on the host or growth
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