A Three-dimensional model reconstruction of pole assembly in Bacillus subtilis

In the rod-shaped bacterium Bacillus subtilis, new polar surfaces arise at division through the centripetal synthesis of a centrally located cross-wall. Subsequently, the cross-wall, analogous to a flat annulus, is converted into two inner layers of polar wall as the daughter cells separate. The junction of polar and cylindrical wall is marked by the presence of raised tears or wall bands formed by the splitting apart of the cross-wall at its base. New polar wall formed in this manner accounts for about 15% of the total surface area. The sequence of pole formation has been simulated by means of a generalized conic section based upon the mathematical rotation of a parabola about its longitudinal axis. Four basic measurements describe the stages of pole formation with reference to polar surface area: the equatorial diameter at the wall bands (D_max), the division furrow (D_min), the horizontal distance (h) from the centre of the cross-wall to D_max and the curvature of the nascent polar surfaces. These four parameters were found to yield a close fit to measurements of polar size and shape derived from electron micrographs of cell poles in sectioned organisms. Calculations of pole curvature suggest that both the initial separation of the cross-wall and separation of the daughter cells may occur very rapidly.