Crossover in the dynamics of cell wall growth controls bacterial division times

Cell size is specific to each species and impacts their ability to function. While various phenomenological models for cell size regulation have been proposed, recent work in bacteria have demonstrated an adder mechanism, in which a cell increments its size by a constant amount between each division. However, the coupling between cell size, shape and constriction, remain poorly understood. Here, we investigate size control and the cell cycle dependence of bacterial growth, using multigenerational cell growth and shape data for single Caulobacter crescentus cells. Our analysis reveals a biphasic growth mechanism: a timer phase before constriction where the cell grows by a constant multiple of its initial size, followed by an adder phase during constriction. Synchronized cell wall labeling measurements reinforce this biphasic behavior: a crossover from uniform lateral growth to localized septal growth is observed. We quantitatively explain this mixer mechanism for size control using a mathematical model.