  We report an in-vivo electromechanical Atomic Force Microscopy (AFM) study of
charge distribution on the cell wall of Gram plus Rhodococcus wratislaviensis
bacteria, naturally adherent to a glass substrate, in physiological conditions.
The method presented in this paper relies on a detailed study of AFM
approach-retract curves giving the variation of the interaction force versus
distance between tip and sample. In addition to classical height and mechanical
(as stiffness) data, mapping of local electrical properties, as bacterial
surface charge, was proved to be feasible at a spatial resolution better than
few tens of nanometers. This innovative method relies on the measurement of the
cantilever's surface stress through its deflection far from (higher than 10nm)
the repulsive contact zone. The variations of surface stress come from
modification of electrical surface charge of the cantilever (as in classical
electrocapillary measurements) likely stemming from its charging during contact
of both tip and sample electrical double layers. This method offers an
important improvement in local electrical and electrochemical measurements at
the solid-liquid interface particularly in high-molarity electrolytes when
compared to technics focused on the direct use of electrostatic force. It thus
opens a new way to directly investigate in-situ biological electrical surface
processes involved in numerous practical and fundamental problems as bacterial
adhesion, biofilm formation, microbial fuel cell, etc.

Soft Condensed Matter;Other Condensed Matter