A fluidization transition of the cell envelope sets up a universal volume expansion pacemaker.

The bacterial cell wall is a complex and resource-intensive biochemical structure that must expand in concert with biomass growth to prevent lysis or molecular crowding. Elegant experiments have shown that cell volume growth continues largely unperturbed in the face of periodic osmotic shocks, raising the question how the pace of cell wall expansion is regulated in E. coli . We discovered a proportional relationship of turgor pressure with growth rate. Remarkably, despite this increase in turgor, cellular biomass density was constant across a wide range of growth rates. In contrast, perturbing turgor away from this scaling directly affected density. To understand these confusing observations, we formulated a mathematical model, in which endopeptidase-mediated cell wall fluidization enables turgor pressure to set the pace of cellular volume expansion. This model not only explains our observations but makes a set of non-trivial predictions that we tested experimentally. As predicted, we found that modulating the effective viscosity of the cell wall was sufficient to control cellular biomass density. We also validated a surprising inverse relationship between cell width and biomass density. A remaining puzzle was what mediates the increase in turgor pressure with growth rate. The picture that emerges is that changes in turgor pressure across growth rates are mediated by changes in the concentration of counterions, balancing the net charge of ribosomal RNA. Profoundly, the coupling between rRNA and pressure simultaneously coordinates cell volume expansion across growth rates and exerts homeostatic feedback control on cytoplasmic density, explaining the rapid adjustment of cell volume to total biomass after perturbations. Physical forces generated via this universal mechanism may drive the expansion of fast-growing, ribosome-dense cells in their local environment, ranging from microbes to aggressive cancers.