Extracellular electron transfer via microbial nanowires

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Abstract
Microbes that can transfer electrons to extracellular electron acceptors, such as Fe(iii) oxides, are important in organic matter degradation and nutrient cycling in soils and sediments1,2. Previous investigations on electron transfer to Fe(iii) have focused on the role of outer-membrane c-type cytochromes1,3. However, some Fe(iii) reducers lack c-cytochromes4. Geobacter species, which are the predominant Fe(iii) reducers in many environments1, must directly contact Fe(iii) oxides to reduce them5, and produce monolateral pili6 that were proposed1,2, on the basis of the role of pili in other organisms7,8, to aid in establishing contact with the Fe(iii) oxides. Here we report that a pilus-deficient mutant of Geobacter sulfurreducens could not reduce Fe(iii) oxides but could attach to them. Conducting-probe atomic force microscopy revealed that the pili were highly conductive. These results indicate that the pili of G. sulfurreducens might serve as biological nanowires, transferring electrons from the cell surface to the surface of Fe(iii) oxides. Electron transfer through pili indicates possibilities for other unique cell-surface and cell–cell interactions, and for bioengineering of novel conductive materials.