Extracellular Electron Transfer by the Gram-Positive Bacterium Enterococcus faecalis

Extracellular electron transfer (EET) in microbial cells is essential for certain biotechnological applications and contributes to the biogeochemical cycling of elements and syntrophic microbial metabolism in complex natural environments. The Gram-positive lactic acid bacterium Enterococcus faecalis, an opportunistic human pathogen, is shown to be able to transfer electrons generated in fermentation metabolism to electrodes directly and indirectly via mediators. By exploiting E. faecalis wild-type and mutant cells, we demonstrate that reduced demethylmenaquinone in the respiratory chain in the bacterial cytoplasmic membrane is crucial for the EET. Heme proteins are not involved, and cytochrome bd oxidase activity was found to attenuate EET. These results are significant for the mechanistic understanding of EET in bacteria and for the design of microbial electrochemical systems. The basic findings infer that in dense microbial communities, such as in biofilm and in the large intestine, metabolism in E. faecalis and similar Gram-positive lactic acid bacteria might be electrically connected to other microbes. Such a transcellular electron transfer might confer syntrophic metabolism that promotes growth and other activities of bacteria in the microbiota of humans and animals.