The mechanistic details of bacterial cell division are poorly understood. The Gram-positive bacterium Bacillus subtilis can divide via two modes. During vegetative growth, the division septum is formed at the mid cell to produce two equal daughter cells. However, during sporulation, the division septum is formed closer to one pole to yield a smaller forespore and a larger mother cell. We use cryo-electron tomography to visualize the architectural differences in the organization of FtsAZ filaments, the major orchestrators of bacterial cell division during these conditions. We demonstrate that during vegetative growth, FtsAZ filaments are present uniformly around the leading edge of the invaginating septum but during sporulation, they are only present on the mother cell side. Our data show that the sporulation septum is thinner than the vegetative septum during constriction, and that this correlates with half as many FtsZ filaments tracking the division plane during sporulation as compared to vegetative growth. We further find that a sporulation-specific protein, SpoIIE, regulates divisome localization and septal thickness during sporulation. Our data provide first evidence of asymmetric localization of the cell division machinery, and not just septum formation, to produce different cell types with diverse fates in bacteria.
3D Imaging of Membrane Networks in a Bacterial Cell Using Electron Tomography