High Osmolarity Modulates Bacterial Cell Size through Reducing Initiation Volume in Escherichia coli

Bacterial cell size is closely associated with biomass growth and cell cycle progression, including chromosome replication and cell division. It is generally proposed that Escherichia coli cells tightly control the timing of chromosome replication through maintaining a constant cell volume per origin upon initiating chromosome replication (constant initiation volume) under various growth conditions. Here, we quantitatively characterize the cell size and cell cycle of Escherichia coli cells growing exponentially under hyperosmotic stress, which is a common environmental stressor that profoundly affects the bacterial water content. The bacterial cell size is reduced by hyperosmotic stress, even though the C and D periods are remarkably prolonged, indicating a significantly reduced initiation volume. The reduced initiation volume originates from the higher concentration of DnaA initiator protein caused by water loss at high osmolarity. Our study shows suggests a fundamental role of water content in regulating bacterial cell size and has also revealed a new role of the DnaA protein in regulating the chromosome replication elongation beyond regulating the replication initiation process. IMPORTANCE Bacterial cell size depends on growth rate, cell cycle progression, and the cell volume per origin upon initiating chromosome replication (initiation volume). Here, we perform the first systematic and quantitative study of the effect of hyperosmotic stress on the E. coli cell size and cell cycle. We find that hyperosmotic stress significantly reduces the initiation volume. The reduced initiation volume is attributed to the increased DnaA concentration caused by water loss at high osmolarity, indicating a fundamental role of water content in cell size and cell cycle regulation.