Progress in electrohydrodynamics of soft microbial particle interphases

Electrokinetic phenomena, such as electrophoresis, are valuable tools for determining the interfacial (double layer) properties of colloidal particles. The theoretical formalisms employed to interpret electrokinetic data (electrophoretic mobility) were initially derived for the restrictive case of hard (non-permeable) particles with the electrokinetic potential as unavoidable primary variable. In this paper, we underline the inadequacy of such models for addressing the electrostatic and hydrodynamic characteristics of microbes like bacteria, viruses or yeast cells. These bioparticles are characterized by heterogeneous, soft, permeable interphases formed with the outer electrolytic medium, which requires advanced electrokinetic analyses where the concept of zeta-potential must be abandoned. We review the progresses made in the measurement and analysis of interphasial properties of bioparticles under electrokinetic conditions. In particular, emphasis is given on the necessity to couple appropriately interpreted electrokinetics with other physico-chemical measurements (e.g. issued from AFM imaging/force spectroscopy) and microbiological techniques (genetic manipulation of microbes). Using such a combination, a clear connection between complex interphase properties of microbes and e.g. their propensity to adhere onto charged surfaces should be achieved.