Going Wireless: Fe(III) Oxide Reduction without Pili in Geobacter sulfurreducens Strain JS-1

The conductive pili of Geobacter sulfurreducens are essential for optimal extracellular electron transfer to Fe(III) and long-range electron transport through current-producing biofilms. The KN400 strain of G. sulfurreducens reduces poorly crystalline Fe(III) oxide more rapidly than the more extensively studied DL-1 strain. Deletion of the gene for PilA, the structural pilin protein, in strain KN400 inhibited Fe(III) oxide reduction. However, slow rates of Fe(III) reduction were detected after extended (> 30 days) incubation in the presence of Fe(III) oxide. After seven consecutive transfers the PilA-deficient strain adapted to reduce Fe(III) oxide as fast as the wild type. Microarray, proteomic, and gene deletion studies indicated that this adaptation was associated with greater production of the c-type cytochrome PgcA, which was released into the culture medium. It is proposed that the extracellular cytochrome acts as an electron shuttle, promoting electron transfer from the outer cell surface to Fe(III) oxides. The adapted PilA-deficient strain competed well with the wild-type strain when both were grown together on Fe(III) oxide. However, when 50% of the culture medium was replaced with fresh medium every three days, the wild-type strain out-competed the adapted strain. A possible explanation for this is that the necessity to produce additional PgcA, to replace the PgcA continually removed, put the adapted strain at a competitive disadvantage, similar to the apparent selection against electron-shuttling producing Fe(III) reducers in most soils and sediments. Despite increased extracellular cytochrome production, the adapted PilA-deficient strain produced low levels of current; consistent with the concept that long-range electron transport through G. sulfurreducens biofilms cannot be achieved without PilA-pili. An eight-chip study using total RNA recovered from four separate cultures of Geobacter sulfurreducens JS-1 (experimental condition) or Geobacter sulfurreducens KN400 (control condition) grown with acetate (10mM)-Fe(III) oxide (100 mmol l-1) exponential growth. Each chip measures the expression level of 3,328 genes from Geobacter sulfurreducens KN400 with nine 45-60-mer probe pairs (PM/MM) per gene, with three-fold technical redundancy.