Bacterial growth rate in populations and single cells.

Bacterial growth is straightforward to measure, complex in origin and exquisitely sensitive to environmental and genetic perturbations. As a quantifiable parameter, growth rate is central to modern microbiology, conflictingly portrayed as both an output and an input of physiological state. Here we provide a short history of bacterial growth measurement and explain the centrality of growth rate to modern microbiology. We then examine the physical and regulatory connections between nutrient availability, macromolecular synthesis and growth rate. We focus particular attention on (p)ppGpp, a nucleotide alarmone that offers the best-characterized mechanism for coordinating environmental conditions with growth. We end by addressing the cell-autonomous nature of homeostatic control as the basis of growth rate heterogeneity, discussing why the individual cell is paramount when it comes to understanding the relationship between different aspects of physiology, including macromolecular composition, cell size, cell cycle progression and growth rate.