ELECTRON MICROSCOPY OF SPORE FORMATION AND GERMINATION IN BACILLUS SUBTILIS

Abstract

The spore formation and germination process in Bacillus subtilis were followed by electron microscopy of ultrathin sections.
At the beginning of sporulation, the cytoplasmic membrane, associated with a small amount of the cell wall material, centripetally invaginates into the cytoplasm at opposite sites near one end of the cell to develop the forespore septum. This septum is composed of two double-layers of the extending cytoplasmic membrane and of a cortical interspace, and it progressively encloses the nuclear apparatus and finally becomes discontinuous with the cytoplasmic membrane. The intracytoplasmic membrane system is present inside as well as outside the forespore, and it appears to connect to the forespore septum and membrane. The cortex is progressively deposited between the opposite layers of the forespore septum and membrane. Only after the completion of the forespore, the initial spore coat appears fragmentarilly as a dense layer in the sporangial cytoplasm peripheral to the outer forespore membrane, and it develops to a multilaminated envelope. As the forespore develops to a mature spore, the nuclear fibrils and the membraneous organelle become invisible in the uniformly homogeneous spore cytoplasm. It seems that the inner layer of the forespore membrane finally develops into the core membrane (probably the spore cytoplasmic membrane) and the outer one gradually disappears.
At the initial stages of spore germination, the cortex swells outward and is transformed to a spongy structure following partial dissolution. Immediately after this, the nuclear fibrils appear in the lighter region within the granulated spore cytoplasm, and successively the cell wall, the cytoplasmic membrane and the membraneous organelle become distinctly visible. As the dissolution of the cortex proceeds, a new vegetative cell is released, rupturing the spore coat near the equatorial portion.