Cytochrome OmcS Is Not Essential for Extracellular Electron Transport via Conductive Pili in Geobacter sulfurreducens Strain KN400

The multi-heme c-type cytochrome OmcS is one of the central components used for extracellular electron transport in the Geobacter sulfurreducens strain DL-1, but its role in other microbes, including other strains of G. sulfurreducens, is currently a matter of debate. Therefore, we investigated the function of OmcS in the G. sulfurreducens strain KN400, which is even more effective in extracellular electron transfer than the DL-1 strain. We found that deleting omcS from strain KN400 did not negatively impact the rate of Fe(III) oxide reduction and that the cells expressed conductive filaments. Replacing the wild-type pilin gene with the aro-5 pilin gene eliminated the OmcS-deficient strain's ability to transport electrons to insoluble electron acceptors and diminished filament conductivity. These results are consistent with the concept that electrically conductive pili are the primary conduit for long-range electron transfer in G. sulfurreducens and closely related species. These findings, coupled with the lack of OmcS homologs in other microbes capable of extracellular electron transfer, suggest that OmcS is not a common critical component for extracellular electron transfer. IMPORTANCE OmcS has been widely studied and noted to be one of the key components for extracellular electron exchange by the Geobacter sulfurreducens strain DL-1. However, the true importance of OmcS warrants further investigation because it is well known that few bacteria, even within the Geobacteraceae family, contain OmcS homologs, and many bacteria that are capable of extracellular electron transfer lack an abundance of any type of outer surface c-type cytochrome. In addition, there is debate about the importance of OmcS filaments in the mechanism of extracellular electron transport to insoluble electron acceptors by G. sulfurreducens. It has been suggested that filaments comprised of OmcS rather than e-pili are the predominant conductive filaments expressed by G. sulfurreducens. However, the results presented here, along with multiple other sources of evidence, indicate that OmcS filaments cannot be the primary, conductive, protein nanowires expressed by G. sulfurreducens.