Self-organization of swimmers drives long-range fluid transport in bacterial colonies

TL;DR

This study reveals that motile bacteria in colonies self-organize into rings that generate fluid flows, facilitating long-range material transport and affecting community structure.

Contribution

It uncovers a novel self-organization mechanism in bacterial colonies where motile cells form rings that drive fluid circulation, a process not previously described.

Findings

Motile bacteria form centimeter-scale rings around colonies.

Self-organized rings induce fluid flows at ~30 microns/sec.

This mechanism influences bacterial community structure and material distribution.

Abstract

Bacteria commonly live in structured communities that affect human health and influence ecological systems. Heterogeneous populations, such as motile and non-motile populations, often coexist in bacteria communities. Motile subpopulations in microbial communities are believed to be important to dispersal, quest for food, and material transport. However, except in circumstances where motile cells drive colony expansion (e.g. bacterial swarming), the physiological functions of motile subpopulations in bacterial communities are largely unclear. Here we discovered that motile cells in routinely cultured sessile colonies of peritrichously flagellated bacteria can self-organize into two adjacent, centimeter-scale motile rings surrounding the entire colony. The motile rings arise from spontaneous segregation of a homogeneous swimmer suspension that mimics a phase separation; the process is mediated by intercellular interactions and shear-induced depletion. As a result of this self-organization, cells drive fluid flows to circulate around the colony at a constant peak speed of approximately 30 microns per second, providing a stable and high-speed avenue for directed material transport at the macroscopic scale. These findings present a unique form of bacterial self-organization that influences population structure and material distribution in bacterial communities.