Terminal organelle development in the cell wall-less bacterium Mycoplasma pneumoniae

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Abstract
Mycoplasmas are cell wall-less bacteria considered among the smallest and simplest prokaryotes known, and yet several species including Mycoplasma pneumoniae have a remarkably complex cellular organization highlighted by the presence of a differentiated terminal organelle, a membrane-bound cell extension distinguished by an electron-dense core. Adhesin proteins localize specifically to the terminal organelle, which is also the leading end in gliding motility. Duplication of the terminal organelle is thought to precede cell division, but neither the mechanism of its duplication nor its role in this process is understood. Here we used fluorescent protein fusions and time-lapse digital imaging to study terminal organelle formation in detail in growing cultures of M. pneumoniae. Individual cells ceased gliding as a new terminal organelle formed adjacent to an existing structure, which then migrated away from the transiently stationary nascent structure. Multiple terminal organelles often formed before cytokinesis was observed. The separation of terminal organelles was impaired in a nonmotile mutant, indicating a requirement for gliding in normal cell division. Examination of cells expressing two different fluorescent protein fusions concurrently established their relative order of appearance, and changes in the fluorescence pattern over time suggested that nascent terminal organelles originated de novo rather than from an existing structure. In summary, spatial and temporal analysis of terminal organelle formation has yielded insights into the nature of M. pneumoniae cell division and the role of gliding motility in that process.