Identification of extracellular matrix components essential for a conductive Geobacter sulfurreducens biofilm

Abstract

Electron transfer from <italic>Geobacter sulfurreducens</italic> cells to electrodes or metal oxides requires proper expression and localization of redox-active proteins as well as attachment mechanisms that interface bacteria with surfaces and other cells. Type IV pili and <italic>c</italic>-type cytochromes have long been considered important components of this conductive network. In this work, a large-scale mutagenesis of <italic>G. sulfurreducens</italic> was performed and mutants were screened for extracellular electron transfer and attachment phenotypes, identifying new genes essential for a conductive <italic>Geobacter</italic> network. A mutant defective in polysaccharide export to the extracellular matrix (&#916;1501, &#916;<italic>xapD::kan</italic>) was identified based on its altered surface attachment. Characterization of this mutant revealed the importance of extracellular polysaccharides for proper attachment and anchoring of the external <italic>c</italic>-type cytochromes necessary for a conductive biofilm network. Furthermore, decreased polysaccharide content was found in commonly studied cytochrome and type IV pili mutants, with defects in cell to cell and cell to surface attachment correlating with levels of extracellular polysaccharides. The extracellular matrix of <italic>G. sulfurreducens</italic> is therefore a complex network of polysaccharides, type IV pili, and <italic>c</italic>-type cytochromes. Disruption of any one of these extracellular components alters overall matrix properties and impedes extracellular electron transfer and attachment.