Title: The Physics of Bacterial Cell Shape and Size
Abstract: All living cells are bounded by envelopes that protect them from the environment and confer their sizes and shapes. These shapes help cells to spatially organize their internal biological processes, allowing them to divide and faithfully segregate genetic material to each daughter. Yet, we still know very little about how cells obtain and control cell shape, even in the arguably simplest and best understood organism: the rod-shaped Escherichia coli. To resist a high intracellular osmotic pressure, bacteria and many other single-celled organisms are surrounded by a cell wall, an elastic, covalent meshwork of sugars and peptides. For walled cells to grow, they must enzymatically cut cell-wall bonds while inserting new cell-wall material to prevent envelope rupture. In some bacteria and many fungi and plants, the process of cell-wall remodeling is limited to the tip of the cell, where intracellular pressure and enzymatic cell-wall fluidization drive growth of the rod, akin to glass blowing. However, in E. coli and many other rod-shaped bacteria, cell-wall remodeling happens all along the cylindrical part of the cell, while the poles remain inert (new poles being constructed at mid-cell during division). How do cells control a straight rod-like cell geometry with a well-defined diameter, all the while increasing cell length at a rate that accommodates biomass growth? While the ultimate answers to these questions remain to be found, we have made important progress in the past two decades. For example, i) curved cytoskeletal polymers sense cell-envelope curvature and reenforce cylindrical geometry, ii) mechanical stress affects envelope growth locally, and iii) the ratio between cell-surface area and biomass emerges as a controlled variable, thus coupling the global rate of envelope growth to the rate of biomass growth. I will present these and other findings, illustrate the importance of experiments and theory, and present future directions.