Probing surface structures of Shewanella spp. by microelectrophoresis

Long-range electrostatic forces substantially influence bacterial interactions and bacterial adhesion during the preliminary steps of biofilm formation. The strength of these forces depends strongly on the structure of the bacterium surfaces investigated. The latter may be addressed from appropriate analysis of electrophoretic mobility measurements. Due to the permeable character of the bacterium wall and/or surrounding polymer layer, bacteria may be regarded as paradigms of soft bioparticles. The electrophoretic motion of such particles in a direct-current electric field differs considerably from that of their rigid counterparts in the sense that electroosmotic flow takes place around and within the soft surface layer. Recent developments of electrokinetic theories for soft particles now render possible the evaluation of the softness degree (or equivalently the hydrodynamic permeability) from the raw electrokinetic data. In this article, the electrophoretic mobilities of three Shewanella strains (MR-4, CN32, and BrY) presenting various and well-characterized phenotypes of polymer fringe are reported over a wide range of pH and ionic strength conditions. The data are quantitatively analyzed on the basis of a rigorous numerical evaluation of the governing electrostatic and hydrodynamic equations for soft particles. It is clearly shown how the peculiar surface structures of the bacteria investigated are reflected in their electrohydrodynamic properties.