Bacteria control cell volume by coupling cell-surface expansion to dry-mass growth

Cells exhibit a high degree of intracellular crowding. To control the level of crowding during growth cells must increase their volumes in response to the accumulation of biomass. Using Escherichia coli as a model organism, we found that cells control cell volume indirectly, by increasing cell-surface area in proportion to biomass growth. Thus, dry-mass density, a readout of intracellular crowding, varies in proportion to the surface-to-volume ratio, both during the cell cycle and during perturbations such as nutrient shifts. On long time scales after shifts, initial dry-mass density is nearly restored by slow variations of the surface-to-mass ratio. Contrary to a long-standing paradigm, cell-envelope expansion is controlled independently of cell-wall synthesis but responds to the activity of cell-wall cleaving hydrolases. Finally, we observed rapid changes of Turgor pressure after nutrient shifts, which were likely responsible for initial changes of cell diameter and dry-mass-density. Together, our experiments reveal important regulatory relationships for cell volume and shape.