![]() Engulfment entails a dramatic reorganization of the sporangium, from two cells that lie side by side to a forespore contained within the cytoplasm of the mother cell. After division, the mother cell engulfs the forespore in a phagocytosis-like manner. The initial step of sporulation is the formation of an asymmetrically positioned septum (polar septation), which produces a larger mother cell and a smaller forespore ( Figure 1A). To survive starvation, the Gram-positive bacterium Bacillus subtilis forms durable endospores ( Tan and Ramamurthi, 2014). ![]() More work will also be needed to understand exactly how bacterial cells coordinate the cell wall synthesis and cell wall degradation. Other spore-forming bacteria that threaten human health – such as Clostridium difficile, which causes bowel infections, and Bacillus anthracis, which causes anthrax – might form their spores in the same way, but this remains to be tested. propose that the junction between the septum and the cell wall moves around the forespore to make room for the mother cell’s membrane for expansion. Running a simple biophysical model on a computer showed that coordinating these two processes could generate enough force for a mother cell to engulf a forespore. The experiments show that, in order to engulf the forespore, the mother cell must produce new cell wall and destroy cell wall that is no longer needed. ![]() To address this question, Ojkic, López-Garrido et al. However, it is not clear how the mother cell can generate the physical force required to engulf the forespore under the cramped conditions imposed by the cell wall. This happens once a partition in the cell wall, called a septum, has formed, separating mother and daughter cells. For this process to take place, a rigid mesh-like layer called the cell wall, which lies outside the cell membrane, needs to be remodelled. Then the membrane that surrounds the mother cell moves to surround the forespore and engulf it. To make a spore, the bacterial cell divides to make a larger mother cell and a smaller forespore cell. Some bacteria, such as Bacillus subtilis, form spores when starved of food, which enables them to lie dormant for years and wait for conditions to improve. Hence, we establish a biophysical mechanism for the creation of a force for engulfment based on the coordination between cell wall synthesis and degradation. We propose a simple model for engulfment in which the junction between the septum and the lateral cell wall moves around the forespore by a mechanism resembling the ‘template model’. Here, we show that membrane migration is driven by cell wall remodeling at the leading edge of the engulfing membrane, with peptidoglycan synthesis and degradation mediated by penicillin binding proteins in the forespore and a cell wall degradation protein complex in the mother cell. ![]() However, the force generation mechanism for forward membrane movement remains unknown. ![]() Subsequently, the mother cell membrane engulfs the forespore in a phagocytosis-like process. Sporulation initiates with an asymmetric cell division, creating a large mother cell and a small forespore. When starved, the Gram-positive bacterium Bacillus subtilis forms durable spores for survival. ![]()
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