Cell Geometry Limits Bacterial Metabolic Efficiency.

Bacterial metabolic strategies are fundamentally linked to their physical form, yet a quantitative understanding of how cell size and shape constrain the efficiency of biomass production remains poorly understood. Here, we develop a coarse-grained whole-cell model of bacterial physiology that integrates proteome allocation, metabolic fluxes, and cell geometry with physical limits on cell surface area and intracellular diffusion. Our model shows that the efficiency of cellular growth is not monotonic with nutrient availability; instead, it peaks precisely at the onset of overflow metabolism, framing this metabolic switch as an optimal tradeoff between efficient use of imported nutrients and rapid growth. By simulating perturbations to cell morphology, we demonstrate the strong metabolic advantage of a high surface-to-volume ratio, which consistently improves growth efficiency. Finally, we show how geometric limits on growth efficiency result in a hard physical constraint: the maximum sustainable cell size is inversely related to the growth rate. This is due to a fundamental conflict between the proteomic cost of growth speed and the cost of size, which creates a budget crisis in large, fast-growing cells. Our work shows how a few physical rules define the allowable strategies for bacterial metabolism and provides a mechanistic explanation for the observed limits on microbial cell size and growth.