Mechanisms underlying rod-shaped cell-wall growth. (a) Top: in rod-shaped bacteria such as E. coli, new cell wall is inserted along the cylindrical midcell (shaded green region) and not at the poles (cyan). Recent evidence suggests that insertion occurs in bursts (green patches) and is coordinated by the bacterial actin homolog MreB. Bottom: the circumferential motion of cytoplasmic MreB polymers (purple) is dependent on cell-wall synthesis, suggesting that MreB tracks represent new glycan strands (green) that have been added into the old wall with peptide crosslinks (red). (b) In tip-growing organisms such as S. pombe, new cell wall is added and remodeled at the growing cell tip(s), and turgor pressure provides force for elongation. Cell-wall synthases and new membrane are targeted to the cell tip by membrane trafficking directed by actin cables that emanate from the cell tips. (c) Stresses in a spherical shell are the same in every direction, while for a thin cylindrical shell the circumferential stress is twice as large as the longitudinal stress (equation 2).