  The population growth of a Staphylococcus aureus culture was followed by
rheological measurements, under steady-state and dynamic shear flows. We
observed a rich viscoelastic behaviour as a consequence of the bacteria
activity. First, the viscosity increased ~10 times due to cell multiplication
and aggregation. This viscosity increase presented several drops and full
recoveries, which are reproducible, allowing us to evoke the existence of a
percolation phenomenon. Eventually, as the bacteria population reached a final
stage of development, fulfilling the sample volume, the viscosity returned to
its initial value, most probably caused by a change in the bacteria
physiological activity, in particular the decrease of their adhesion
properties. Finally, the viscous and the elastic moduli presented power law
behaviours compatible with the "soft glassy materials" model, which exponents
are dependent on the bacteria growth stage.
