Collective motion and density fluctuations in bacterial colonies

Author :H. P. Zhang, Avraham Be'er, E.-L. Florin, and Harry L. Swinney
Publication :PNAS
Volume :107
Number :31
Pages :13626-13630
Year :2010

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.