  Micro-organisms aggregate through chemotaxis against a concentration gradient
of signals secreted by themselves. We have numerically studied a model
consisting of elements with intracellular dynamics, random walks with a
state-dependent turnover rate, and secretion of attractant. Three phases with
and without aggregation, as well as partial aggregation, were obtained as to
the diffusion and decomposition rates of the attractant, and conditions for
cellular aggregation were analyzed. The size of aggregated clusters was shown
to be independent of cell density, as is consistent with experiment.
