DNA–Protein Stoichiometry as a Determinant of Bacterial Growth Rate

Current models of cell size homeostasis mechanisms dictate that cells add a constant volume during each cell cycle and grow at a constant exponential rate. However, this does not fully explain the size and growth rate variability observed among cells, specifically sister cells. With tools such as the mother machine, we are able to examine growth rate more closely, allowing a more nuanced look into size control mechanisms. In particular, we see that when a mother cell divides asymmetrically, the smaller sister tends to grow faster than its larger counterpart, adding more volume in the following cycle and making up for the difference in birth size. We can explain this varied growth rate to a difference in concentration of DNA as compared to other cellular components. This implies that the growth rate within a single cell cycle is not a constant exponential, but instead varies with the changing concentration of DNA as the cell grows. In order to examine this relationship more closely, we expand the dynamic range of DNA concentration in living cells by inducing filamentation with two antibiotics: one that inhibits DNA synthesis, and one that leaves that process unchanged. Our measurements of DNA-protein ratio and instantaneous growth rate show a direct relationship, offering insight into a more complex size homeostasis mechanism that compensates for size fluctuations stemming from asymmetrical division.