Lawrence Berkeley National Laboratory

The purpose of this study is to understand the distinct shape profile of the dividing Caulobacter crescentus (Cc) cell near the division midplane at various stages of cell division, visible in new high-resolution Cryo EM tomographic images. These images of Cc during cell division provide the needed resolution for understanding cell wall dynamics under the influence of the contractile ring at the division midplane (FtsZ ring). The bacterial cell wall is a peptidoglycan mesh that acts like a fabric that can support shear as well as distinct normal forces, as opposed to the lipid membranes which are usually assumed not to support shear or any differences in normal forces. New tomographic images clearly show the S-layer, inner, outer membranes and peptidoglycan mesh. The shape of the cell wall during division and the deflection at the division plane is a function of the material properties of the cell wall, the growth rate, and the force due to the FtsZ ring. Our hypothesis is that cell growth and cell contraction occur on similar timescales, and that the contractile force at the division midplane is minimal, serving to direct the growth of the peptidoglycan mesh very near the division plane so as to eventually pinch and isolate the two halves of the dividing cell. In this model the potential energy stored in the peptidoglycan mesh under the strain induced by the FtsZ ring is always minimal and the insertion of new cell wall material near division midplane at the point of inflection in the dividing cell is essential. Using this model we can predict the force due to the contractile ring given specific elastic parameters and growth rate.