  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.
