A Constant Size Extension Drives Bacterial Cell Size Homeostasis

Cell size control is an intrinsic feature of the cell cycle. In bacteria, cell growth and division are thought to be coupled through a cell size threshold. Here, we provide direct experimental evidence disproving the critical size paradigm. Instead, we show through single-cell microscopy and modeling that the evolutionarily distant bacteria Escherichia coli and Caulobacter crescentus achieve cell size homeostasis by growing, on average, the same amount between divisions, irrespective of cell length at birth. This simple mechanism provides a remarkably robust cell size control without the need of being precise, abating size deviations exponentially within a few generations. This size homeostasis mechanism is broadly applicable for symmetric and asymmetric divisions, as well as for different growth rates. Furthermore, our data suggest that constant size extension is implemented at or close to division. Altogether, our findings provide fundamentally distinct governing principles for cell size and cell-cycle control in bacteria. [Display omitted] •Bacterial cell elongation over a cell cycle is independent of cell length at birth•To control their size, cells sense how much they have grown, not how big they are•The constant size extension occurs between divisions•This mechanism is extremely robust and broadly applicable To achieve cell size homeostasis, bacteria elongate by a constant amount between divisions, irrespective of size at birth. This mechanism, relying on the ability of cells to sense how much they have grown and not how big they are, provides new governing rules for cell-cycle control.