Bacteria exhibit a high degree of intracellular macromolecular crowding. To control the level of crowding cells must increase their volumes in response to the accumulation of biomass during growth. This coordination is fundamentally not understood in any cell type. Using rod-shaped bacteria Escherichia coli and Bacillus subtilis as a model organism, we found that cells control volume indirectly, by robustly increasing cell-surface area rather than volume in proportion to biomass. Therefore, dry-mass density changes with cell shape, both during the cell cycle and during nutrient shifts, in direct proportion to the surface-to-volume ratio.

The major contribution to mass-density variations during nutrient shifts comes from changes of cell width. We found that these variations are likely driven mechanically, through nutrient-dependent changes of Turgor pressure. Thus, we identified cell dimensions and a new, slowly varying surface-to-mass coupling constant as the sole independent variables responsible for the coordination of volume with mass growth.

Through a range of perturbations and the analysis of single-cell correlations we elucidate potential mechanisms underlying surface-to-mass coupling and adaptation. Together, our experiments reveal important regulatory relationships underlying crowding and envelope homeostasis.