Growth rate and cell size: a re-examination of the growth law

• Bacterial size and chemical composition are thought to be growth rate-dependent.
• The correlation between growth rate and size is affected by nutrient availability.
• Nutrient-dependent regulators target the cytokinetic machinery.
• Disruption of biosynthetic pathways may uncouple size from growth rate.
• Cell size is a function of the rates of cell expansion and cell cycle progression.

Research into the mechanisms regulating bacterial cell size has its origins in a single paper published over 50 years ago. In it Schaechter and colleagues made the observation that the chemical composition and size of a bacterial cell is a function of growth rate, independent of the medium used to achieve that growth rate, a finding that is colloquially referred to as ‘the growth law’. Recent findings hint at unforeseen complexity in the growth law, and suggest that nutrients rather than growth rate are the primary arbiter of size. The emerging picture suggests that size is a complex, multifactorial phenomenon mediated through the varied impacts of central carbon metabolism on cell cycle progression and biosynthetic capacity.