Fluctuations in growth rates determine the generation time and size distributions of E. coli cells

Isogenic Escherichia coli growing exponentially in a constant environment display large variation in growth-rates, division-sizes and generation-times. It is unclear how these seemingly random cell cycles can be reconciled with the precise regulation required under conditions where the generation time is shorter than the time to replicate the genome. Here we use single molecule microscopy to map the location of the replication machinery to the division cycle of individual cells. We find that the cell-to-cell variation in growth rate is sufficient to explain the corresponding variation in cell size and division timing assuming a simple mechanistic model. In the model, initiation of chromosome replication is triggered at a fixed volume per origin region, and associated with each initiation event is a division event at a growth rate dependent time later. The result implies that cell division in E. coli has no other regulation beyond what is needed to initiate DNA replication at the right volume.