Collective motion and density fluctuations in
bacterial colonies
Proceedings of the National Academy of Sciences, 107 (31):
13626--13630 (2010)DOI: 10.1073/pnas.1001651107
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.
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