Geobacter sulfurreducens is an electrogenic bacterium capable of oxidizing organic compounds and transferring released electrons into a conductive substrate. Electrogenic bacteria are actively researched for the development of microbial fuel cells, which produce electric energy by oxidizing organic compounds. Such microbial fuel cells are thought to generate electricity by oxidizing organic contaminations, for example in communal waste waters. From all known pure cultures, G. sulfurreducens demonstrated the highest current density in this process.
The development of microbial fuel cells requires an insight into the process of bacterial adsorption on conductive substrates, their subsequent division and the growth of a biofilm, a complex microscopically thick structure built by multiple layers of bacterial cells. Starting from the adherence of first bacteria, the biofilm growth is strongly affected by the morphology and chemical composition of the substrate. Of particular interest are studies of biofilm growth on electrodes modified with various functionalized monolayers.
Not only the morphology of the substrate, but also the bacteria on bare or monolayer-modified substrates can be visualized by AFM, from the first bacterial cells attached. In contrast to other microscopic techniques, such as scanning electron microscopy, AFM does not require treatments (glutaraldehyde fixing, gold sputtering) that inevitably kill examined bacteria. Using AFM, the evolution of biofilm can be studied on one sample. Moreover, in situ AFM imaging of bacteria on a substrate immersed in solution can give access to online monitoring of bacterial adhesion, division and the growth of the biofilm.
The figure on the left shows an AFM image of G. sulfurreducens electrochemically deposited on a gold substrate. The sample was taken out from the solution, allowed to dry and imaged by a Nanosurf FlexAFM in ambient air without any pretreatment. Besides individual and groups of bacteria adsorbed directly on gold surface, bacterial cells adhered on top of other bacteria are visible. The latter represents the first step in the bacterial biofilm formation.
Data courtesy of Dr. I. Pobelov and M. Füeg, University of Berne
Nanosurf Application Note AN00962