Towards a physical understanding of volume regulation in bacteria

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. Using Escherichia coli as a model bacterium we found that cell-to-cell variations in dry-mass density, a read-out of intracellular crowding, can be smaller than 3% during steady-state growth. At the same time we found that dry-mass density shows systematic variations as a function of cell dimension in constant metabolic environments or during rapid changes of growth conditions while the ratio between cell-surface area and dry mass remained nearly constant on the time scale of the cell cycle. We thus concluded that crowding homeostasis is achieved indirectly by coupling the rate of cell-surface expansion directly to the rate of dry-mass growth. On long time scales, cells adjust the rate of surface growth to changes of cellular dimensions, thus reducing variations of mass density between conditions. Correlations at the single-cell level support a physical model of crowding-based feedback on cell-envelope expansion.

In search for a mechanistic origin underlying the coupling between surface growth and biomass growth, we found that cell-envelope expansion is physically governed by cell-wall cleavage but independent of cell-wall synthesis. Furthermore, disruption of membrane synthesis led to rapid arrest of cell-surface expansion.

Together, our experiments reveal important regulatory relationships underlying crowding and envelope homeostasis.