**Collective motion in bacterial colonies
**

Hepeng Zhang

Center for Nonlinear Dynamics and Department of Physics

The University of Texas at Austin, Austin

We study collective motion in two species of swarming bacteria.
In colonies of Paenibacillus dendritiformis, the characteristic
correlation length of collective motion is essentially independent
of the mean bacterial speed, while the correlation time is linearly
proportional to the ratio of the correlation length to the mean
speed. In experiments with Bacillus subtilis, we observe that
bacteria swim cooperatively in dynamic clusters, whose size exhibits
a power-law distribution truncated by an exponential tail. Large
mobile clusters lead to anomalous fluctuations in bacterial density;
the standard deviation (N) grows with the mean (N) of the number of
bacteria as N ~ N^3/4 rather than N ~ N^1/2.