  In some conditions, bacteria self-organise into biofilms, supracellular
structures made of a self-produced embedding matrix, mainly composed on
polysaccharides, DNA, proteins and lipids. It is known that bacteria change
their colony/matrix ratio in the presence of external stimuli such as
hydrodynamic stress. However, little is still known about the molecular
mechanisms driving this self-adaptation. In this work, we monitor structural
features of Pseudomonas fluorescens biofilms grown with and without
hydrodynamic stress. Our measurements show that the hydrodynamic stress
concomitantly increases the cell density population and the matrix production.
At short growth timescales, the matrix mediates a weak cell-cell attractive
interaction due to the depletion forces originated by the polymer constituents.
Using a population dynamics model, we conclude that hydrodynamic stress causes
a faster diffusion of nutrients and a higher incorporation of planktonic
bacteria to the already formed microcolonies. This results in the formation of
more mechanically stable biofilms due to an increase of the number of
crosslinks, as shown by computer simulations. The mechanical stability also
lies on a change in the chemical compositions of the matrix, which becomes
enriched in carbohydrates, known to display adhering properties. Overall, we
demonstrate that bacteria are capable of self-adapting to hostile hydrodynamic
stress by tailoring the biofilm chemical composition, thus affecting both the
mesoscale structure of the matrix and its viscoelastic properties that
ultimately regulate the bacteria-polymer interactions.
