On the growth and form of bacterial colonies

Bacteria are single-celled organisms that inhabit almost every ecosystem on Earth. To overcome challenges in their typically stressful and dynamic natural habitats, bacteria can assemble into macroscopic multicellular aggregates, adopting a structured, communal lifestyle that differs starkly from that of free-living, planktonic cells. Characterization of natural environments suggests that growth in dense aggregates is the primary lifestyle for most bacteria, and in recent years controlled laboratory studies have connected physiological behaviours that are well studied in liquid culture to communal behaviours in bacterial colonies. These increasingly common findings support the idea that many microbial behaviours are best understood in the context of dense aggregates. In this Review, we discuss biophysical studies of the growth and development of such aggregates. We aim to motivate joint experimental and theoretical investigation of the biological and physical underpinnings of communal behaviours within spatially structured bacterial communities. Most bacteria exist in dense aggregates, yet this lifestyle is relatively poorly understood compared with planktonic cultures. This Review explores biophysical models of aggregate development, and how models can be extended to account for the complex behaviours of single-species and multispecies colonies.