Microscopic studies on Thermosipho globiformans implicate a role of the large periplasm of Thermotogales

Thermosipho globiformans is a member of Thermotogales, which contains rod-shaped, Gram-negative, anaerobic (hyper)thermophiles. These bacteria are characterized by an outer sheath-like envelope, the toga, which includes the outer membrane and an amorphous layer, and forms large periplasm at the poles of each rod. The cytoplasmic membrane and its contents are called cell, and the toga and its contents rod, to distinguish between them. Optical cells were constructed to observe binary fission of T. globiformans. High-temperature microscopy of rods adhering to optical cells' coverslips showed that the large periplasm forms between newly divided cells in a rod, followed by rod fission at the middle of the periplasm, which was accompanied by a sideward motion of the newly generated rod pole(s). Electron microscopic observations revealed that sessile rods grown on a glass plate have nanotubes adhered to the glass, and these may be involved in the sideward motion. Epifluorescence microscopy with a membrane-staining dye suggested that formation of the septal outer membrane is distinct from cytokinesis. Transmission electron microscopy indicated that the amorphous layer forms in the periplasm between already-divided cells. These findings suggest that the large periplasm is the structure in which the septal toga forms, an event separate from cytokinesis.