![]() ![]() This detailed engulfment mechanism is an important advance in the understanding of bacterial spore formation. could also see that the mother cell’s membrane formed finger-like projections as it moved around the forespore, something that was not visible with previous techniques. This includes a cycle of peptidoglycan production and degradation that accompanies the advancing mother cell membrane as it surrounds the forespore during engulfment. Comparing these images to cryo-ET images of cells treated with drugs that block the production of peptidoglycan suggested a new engulfment mechanism. These images revealed that the peptidoglycan wall separating the mother cell from forespore is not completely degraded: a thin layer of peptidoglycan persists. generated high resolution images, which provided a look at forespore engulfment in unprecedented detail. By combing cryo-ET with another methodology that allowed them to focus in on thin sections of their sample, Khanna et al. Cryo-ET requires thin samples, thinner even than most bacteria. In cryo-ET, samples are cooled to low temperatures and then imaged with a beam of electrons. subtilis using a technique called cryo-electron tomography (or cryo-ET for short). Microbiologists had thought that all the rigid peptidoglycan must be degraded to allow the partition to deform flexibly during engulfment however, no one had yet observed the tiny structures involved. Engulfment of the spore by the mother cell requires a dramatic change in the shape of this partition. At the beginning of sporulation, the forespore is separated from the mother cell by a peptidoglycan wall. This wall sits outside of the bacterium’s thin flexible membrane and determines the bacterium’s shape. The forespore matures into a hardy spore, which is able to survive in harsh environments and only transform into an active bacterium when conditions improve.īacterial cells are surrounded by a stiff layer of a material called peptidoglycan. One such process is bacterial sporulation: in stressful conditions, bacteria like Bacillus subtilis can divide to produce a smaller cell called a forespore, which the larger mother cell then engulfs. Much of what happens in biology occurs at scales so small that the microscopy methods traditionally used by biologists cannot visualize them. We propose that a limited number of SpoIIDMP complexes tether to and degrade the peptidoglycan ahead of the engulfing membrane, generating an irregular membrane front. Then, the mother cell membrane migrates around the forespore in tiny finger-like projections, whose formation requires the mother cell SpoIIDMP protein complex. Instead, the septum is uniformly and only slightly thinned as it curves towards the mother cell. We show that the septal peptidoglycan is not completely degraded at the onset of engulfment. Subsequently, the mother cell engulfs the forespore. During sporulation, an asymmetrically-positioned septum generates a larger mother cell and a smaller forespore. Here, we visualize Bacillus subtilis sporulation using cryo-electron tomography coupled with cryo-focused ion beam milling, allowing the reconstruction of native-state cellular sections at molecular resolution. The study of bacterial cell biology is limited by difficulties in visualizing cellular structures at high spatial resolution within their native milieu. ![]()
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