Mycoplasma genetic variation and its implication for pathogenesis.

Several pathogenic mycoplasma species are known etiologic agents of diseases in man and animals, which typically involve the respiratory tract, urogenital tract and joints and often show chronicity. Although the basis for this chronicity is not well understood, it is apparent that several species of pathogenic mycoplasmas are endowed with a sophisticated genetic machinery for altering their surface attributes. This surface phenotypic variation is thought to play a key role in the establishment and persistence of mycoplasma infections by enabling evasion of host defences and by ensuring adaptation to the rapidly changing microenvironmental conditions encountered in the host. The variability of mycoplasma surface characteristics results both from reversible ON- and OFF-switching of distinct membrane surface proteins (phase variation), from structural changes of these proteins (size variation) and from changes in their surface presentation (epitope masking and demasking). The majority of these surface proteins that are subject to variation are encoded by multiple variant single-copy genes and are lipid-modified proteins which represent the major coat proteins and surface antigens of several pathogenic mycoplasmas. Variable surface lipoproteins play an important role in the pathogenesis of a mycoplasma infection by providing escape from immune response, and probably by influencing both colonization of and translocation across the mucosal barrier. In this minireview, recent developments regarding the genetic mechanisms and the functional significance of surface lipoprotein variation in the pathogenesis of mycoplasma infections are summarized.