Relative Rates of Surface and Volume Synthesis Set Bacterial Cell Size

•Maintenance of a condition-dependent surface area to volume ratio (SA/V) sets bacterial size•Rates of volume and surface growth both scale with volume, producing SA/V homeostasis•Biosynthesis of surface material in the cytoplasm links surface growth rate to volume•A surface material accumulation threshold for division could underlie length control

SummaryMany studies have focused on the mechanisms underlying length and width determination in rod-shaped bacteria. Here, we focus instead on cell surface area to volume ratio (SA/V) and demonstrate that SA/V homeostasis underlies size determination. We propose a model whereby the instantaneous rates of surface and volume synthesis both scale with volume. This model predicts that these relative rates dictate SA/V and that cells approach a new steady-state SA/V exponentially, with a decay constant equal to the volume growth rate. To test this, we exposed diverse bacterial species to sublethal concentrations of a cell wall biosynthesis inhibitor and observed dose-dependent decreases in SA/V. Furthermore, this decrease was exponential and had the expected decay constant. The model also quantitatively describes SA/V alterations induced by other chemical, nutritional, and genetic perturbations. We additionally present evidence for a surface material accumulation threshold underlying division, sensitizing cell length to changes in SA/V requirements.

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