Genetic Mechanisms of Surface Variation

The mycoplasma cell surface is unique among prokaryotes and is highly evolved to accommodate the lifestyle of these diverse organisms during interaction with their respective vertebrate and plant hosts. Particularly in the absence of a cell wall, several adaptive features must be in place to support the survival of these organisms during transmission and residence in varied niches, both extracellular and in some cases within host cells (22, 24, 68). As with any pathogenic organism, the mycoplasma surface reflects the functional interaction of individual organisms with host compartments, including interactions with the innate and adaptive immune systems. Quite unlike other bacteria, however, all functions are expressed from remarkably limited gene sets that are rapidly becoming delineated through mycoplasma genome sequencing projects (11, 26, 28, 36). Insights from these projects as well as many experimental studies now reveal one striking aspect of mycoplasmal biology: the prevalence of mutation-based systems as an adaptive strategy for survival. In contrast to their eubacterial counterparts, mycoplasmal genomes so far have revealed few of the complex systems for classic gene regulation and environmental sensing that endow adaptive flexibility for individual bacterial cells.