How cells control their size is an important open question. Cell-size homeostasis has been discussed in the context of two major paradigms: “sizer,” in which the cell actively monitors its size and triggers the cell cycle once it reaches a critical size, and “timer,” in which the cell attempts to grow for a specific amount of time before division. These paradigms, in conjunction with the “growth law” and the quantitative bacterial cell-cycle model, inspired numerous theoretical models and experimental investigations, from growth to cell cycle and size control. However, experimental evidence involved difficult-to-verify assumptions or population-averaged data, which allowed different interpretations or limited conclusions. In particular, population-averaged data and correlations are inconclusive as the averaging process masks causal effects at the cellular level.
A recent paper monitors hundreds of thousands of Gram-negative Escherichia coli and Gram-positive Bacillus subtilis cells under a wide range of steady-state growth conditions. The results and demonstrate that cells add a constant volume each generation, irrespective of their newborn sizes, conclusively supporting the so-called constant Δ model. Bacteria (and probably other cells) don’t double in mass before dividing. Instead they add a constant volume (or mass) no matter what their initial size. A small cell adds the same volume as a large cell. By following this rule a cell population quickly converges on a common size.