Despite the central role of division in bacterial physiology, how division proteins work together as a nanoscale machine to divide the cell remains poorly understood. Cell division by cell wall synthesis proteins is guided by the cytoskeleton protein FtsZ, which assembles at mid-cell as a dense Z-ring formed of treadmilling filaments. However, although FtsZ treadmilling is essential for cell division, the function of FtsZ treadmilling remains unclear. Here, we systematically resolve the function of FtsZ treadmilling across each stage of division in the Gram-positive model organism Bacillus subtilis using a combination of nanofabrication, advanced microscopy, and microfluidics to measure the division-protein dynamics in live cells with ultrahigh sensitivity. We find that FtsZ treadmilling has two essential functions: mediating condensation of diffuse FtsZ filaments into a dense Z-ring, and initiating constriction by guiding septal cell wall synthesis. After constriction initiation, FtsZ treadmilling has a dispensable function in accelerating septal constriction rate. Our results show that FtsZ treadmilling is critical for assembling and initiating the bacterial cell division machine.@en

Light microscopy is indispensable for analysis of bacterial spatial organization, yet the sizes and shapes of bacterial cells pose unique challenges to imaging. Bacterial cells are not much larger than the diffraction limit of visible light, and many species have cylindrical shapes and so lie flat on microscope coverslips, yielding low-resolution images when observing their short axes. In this protocol, we describe a pair of recently developed methods named VerCINI (vertical cell imaging by nanostructured immobilization) and µVerCINI (microfluidic VerCINI) that greatly increase spatial resolution and image quality for microscopy of the short axes of bacteria. The concept behind both methods is that cells are imaged while confined vertically inside cell traps made from a nanofabricated mold. The mold is a patterned silicon wafer produced in a cleanroom facility using electron-beam lithography and deep reactive ion etching, which takes ~3 h for fabrication and ~12 h for surface passivation. After obtaining a mold, the entire process of making cell traps, imaging cells and processing images can take ~2–12 h, depending on the experiment. VerCINI and µVerCINI are ideal for imaging any process along the short axes of bacterial cells, as they provide high-resolution images without any special requirements for fluorophores or imaging modalities, and can readily be combined with other imaging methods (e.g., STORM). VerCINI can easily be incorporated into existing projects by researchers with expertise in bacteriology and microscopy. Nanofabrication can be either done in-house, requiring specialist facilities, or outsourced based on this protocol.@en