  Microscopic organisms, such as bacteria, have the ability of colonizing
surfaces and developing biofilms that can determine diseases and infections.
Most bacteria secrete a significant amount of extracellular polymer substances
that are relevant for biofilm stabilization and growth. In this work, we apply
computer simulation and perform experiments to investigate the impact of
polymer size and concentration on early biofilm formation and growth. We
observe as bacterial cells formed loose, disorganized clusters whenever the
effect of diffusion exceeded that of cell growth and division. Addition of
model polymeric molecules induced particle self-assembly and aggregation to
form compact clusters in a polymer size- and concentration-dependent fashion.
We also find that large polymer size or concentration lead to the development
of intriguing stripe-like and dendritic colonies. The results obtained by
Brownian dynamic simulation closely resemble the morphologies that we
experimentally observe in biofilms of a Pseudomonas Putida strain with added
polymers. The analysis of the Brownian dynamic simulation results suggests the
existence of a threshold polymer concentration that distinguishes between two
growth regimes. Below this threshold, the main force driving polymer-induced
compaction is hindrance of bacterial cell diffusion, while collective effects
play a minor role. Above this threshold, especially for large polymers,
polymer-induced compaction is a collective phenomenon driven by depletion
forces. Well above this concentration threshold, severely limited diffusion
drives the formation of filaments and dendritic colonies.
