Abstract ‘ We determine and relate the characteristic velocity, length, and time scales for
bacterial motion in swarming colonies of Paenibacillus dendritiformis growing on semi-solid agar
substrates. The bacteria swim within a thin fluid layer, and they form long-lived jets and vortices.
These coherent structures lead to anisotropy in velocity spatial correlations and to a two-step
relaxation in velocity temporal correlations. The mean squared displacement of passive tracers
exhibits a short-time regime with nearly ballistic transport and a diffusive long-time regime.
We find that various definitions of the correlation length all lead to length scales that are,
surprisingly, 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.