Collective motion and density fluctuations in bacterial colonies

TLDR

Collective motion, exemplified by flocks, schools, and swarms, influences phenomena such as disease spread, yet detailed experimental data remain scarce, limiting model validation. We simultaneously tracked up to a thousand Bacillus subtilis bacteria in a colony, recording their positions, velocities, and orientations over time. Bacteria form densely packed, cooperatively moving clusters whose sizes follow a truncated power‑law, grow with density, and exhibit giant number fluctuations far exceeding thermal equilibrium, demonstrating that bacteria provide a biological platform for studying collective motion.

Abstract

Flocking birds, fish schools, and insect swarms are familiar examples of collective motion that plays a role in a range of problems, such as spreading of diseases. Models have provided a qualitative understanding of the collective motion, but progress has been hindered by the lack of detailed experimental data. Here we report simultaneous measurements of the positions, velocities, and orientations as a function of time for up to a thousand wild-type Bacillus subtilis bacteria in a colony. The bacteria spontaneously form closely packed dynamic clusters within which they move cooperatively. The number of bacteria in a cluster exhibits a power-law distribution truncated by an exponential tail. The probability of finding clusters with large numbers of bacteria grows markedly as the bacterial density increases. The number of bacteria per unit area exhibits fluctuations far larger than those for populations in thermal equilibrium. Such “giant number fluctuations” have been found in models and in experiments on inert systems but not observed previously in a biological system. Our results demonstrate that bacteria are an excellent system to study the general phenomenon of collective motion.