  In the growth of bacterial colonies, a great variety of complex patterns are
observed in experiments, depending on external conditions and the bacterial
species. Typically, existing models employ systems of reaction-diffusion
equations or consist of growth processes based on rules, and are limited to a
discrete lattice. In contrast, the two-dimensional model proposed here is an
off-lattice simulation, where bacteria are modelled as rigid circles and
nutrients are point-like, Brownian particles. Varying the nutrient diffusion
and concentration, we simulate a wide range of morphologies compatible with
experimental observations, from round and compact to extremely branched
patterns. A scaling relationship is found between the number of cells in the
interface and the total number of cells, with two characteristic regimes. These
regimes correspond to the compact and branched patterns, which are exhibited
for sufficiently small and large colonies, respectively. In addition, we
characterise the screening effect observed in the structures by analysing the
multifractal properties of the growth probability.
