  Colonies of rod-shaped bacteria constitute a system of colloidal active
matter with nematic properties. As a single initial bacterium multiplies
through repeated divisions, the resulting colony quickly loses long-range
orientational order, but retains locally ordered domains. At the boundaries of
these domains, topological defects emerge, which move around randomly as the
colony grows. In both experiments and simulations, we find that these defects
are created at a rate that corresponds to the exponential growth of the colony,
resulting in a stable defect density. Both this defect density and the colony's
correlation length are regulated by the aspect ratio of the rod-shaped
particles. Moreover, we find that the defect dynamics are well described by a
Gamma distribution, which is due to repeated divisions and subsequent
re-orientations of the bacteria.
