Stringent Control Over Cytoplasmic and Membrane Densities Defines Regulation of Cell Geometry in Escherichia coli

Microbes exhibit precise control over their composition and geometry in order to adapt and grow in diverse environments. However, the mechanisms that orchestrate this simultaneous regulation, and how they are causally linked, remains poorly understood. In this work, we derive and experimentally test a biophysical model of cell size regulation in Escherichia coli which relates the cellular surface-to-volume ratio to the total macromolecular composition and partitioning of the proteome between cellular compartments. Central to this model is the observation that the macromolecular density of the cytoplasm and the protein density within the cell membranes are maintained at a constant ratio across growth conditions. Using quantitative mass spectrometry, single-cell microscopy, and biochemical assays, we show this model quantitatively predicts a non-linear relationship between the surface-to-volume ratio, proteome localization, and the total ribosome content of the cell. This model holds under perturbations of intracellular ppGpp concentrations-thereby changing the ribosomal content-demonstrating that cellular geometry is not strictly determined by the cellular growth rate. These findings provide a biophysical link between the coregulation of proteome organization and cellular geometry, offering a quantitative framework for understanding bacterial size regulation across conditions.