Multiscale dynamics of the cell envelope of Shewanella putrefaciens as a response to pH change.

The bacterial surface properties of gram-negative Shewanella putrefaciens were characterized by microbial adhesion to hydrocarbons (MATH), adhesion to polystyrene dishes, and electrophoresis at different values of pH and ionic strength. The bacterial adhesion to these two apolar substrates shows significant variations according to pH and ionic strength. Such behavior could be partly explained by electrostatic repulsions between bacteria and the solid or liquid interface. However, a similar trend was also observed at rather high ionic strength where electrostatic interactions are supposed to be screened. The nanomechanical properties at pH 4 and 10 and at high ionic strength were investigated by using atomic force microscopy (AFM). The indentation curves revealed the presence of a polymeric external layer that swells and softens up with increasing pH. This suggests a concomitant increase of the water permeability and so did of the hydrophilicity of the bacterial surface. Such evolution of the bacterial envelope in response to changes in pH brings new insight to the pH dependence in the bacterial adhesion tests. It especially demonstrates the necessity to consider the hydrophobic/hydrophilic surface properties of bacteria as not univocal for the various experimental conditions investigated.