Septation and filament splitting in Methanospirillum hungatei

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Abstract
Methanospirillum hungatei GP1 consists of chains of rod-shaped cells separated from one another by "cell spacers" comprised of two spacer plugs sandwiching a loose, amorphous material. The chain is encased within a highly ordered sheath to form a cylindrical, multicelled filament about 5 to 10 cells long under our growth conditions. Cells within the filament divide by septation in a manner similar to gram-positive eubacteria; the plasma membrane and wall grow inward to partition the cell in two. Yet, unlike gram-positive eubacteria, the wall is flexible, since cells round up when extruded from the sheath; the shape-maintaining structures are the sheath and spacer plugs. After septation and daughter cell separation, the cell spacer grows between the new cells. Initially, the growth of a spacer plug is detected by electron microscopy as the addition of electron-dense layers, exhibiting an 18.0-nm periodicity, at the surface of one new cell pole. Usually three layers develop at this pole before plug assembly is initiated at the opposite pole. As assembly proceeds, the two newly formed plugs separate from each other to form the loose, amorphous central zone of the spacer. Presumably, cell and cell spacer elongation requires sheath extension, since filament growth is observed. The amorphous cell spacer zone continues to expand as the spacer grows larger until lesions appear in the sheath near the zone's midpoint. Usually, the largest spacer zones are found towards each filament's centre and the lesions split the chain in two. Consequently, M. hungatei requires two separate events for filament division: cell replication which is a septation process and filament splitting which is a "cell spacer" breakage.