By Irene Hsiao
What determines when a cell divides is a long standing question. Recently, evidence has emerged that Escherichia coli and a number of other single-celled organisms divide when their size increases by a set amount, but the physical origins of this rule have remained elusive. In a study published in Nature Microbiology on 24 July 2017, Norbert Scherer, Aaron Dinner, and colleagues use a novel combination of imaging and microfluidic techniques that they developed for precise and quantitative measurements of cell shape to conclude that crescent-shaped cells of Caulobacter crescentus exhibit two distinct phases of growth: a phase in which cells add material to their cell walls uniformly, and a second phase in which new growth is concentrated at the site of constriction. Their study suggests a new way for constriction to proceed: rather than using molecular motors to pinch the cell wall, the cell wall is directed to grow in a curved manner to form a pair of oppositely facing hemispherical caps that become the ends of the new pair of cells. This mechanism is much less energetically costly than pinching because a cell wall has the material consistency of hard plastic. This new paradigm motivates further investigation of the molecular basis of cellular shape, size, and division.