Two Routes for Extracellular Electron Transfer in Enterococcus faecalis

Abstract

Enterococcus faecalis cells are known to have ferric reductase activity and the ability to transfer electrons generated in metabolism to the external environment. We have isolated mutants defective in ferric reductase activity and studied their electron transfer properties to electrodes mediated by ferric ions and an osmium complex-modified redox polymer (OsRP). Electron transfer mediated with ferric ions and ferric reductase activity were both found to be dependent on the membrane-associated Ndh3 and EetA proteins, consistent with findings in Listeria monocytogenes. In contrast, electron transfer mediated with OsRP was independent of these two proteins. Quinone in the cell membrane was required for the electron transfer with both mediators. The combined results demonstrate that extracellular electron transfer from reduced quinone to ferric ions and to OsRP occurs via different routes in the cell envelope of E. faecalis. IMPORTANCE The transfer of reducing power in the form of electrons, generated in the catabolism of nutrients, from a bacterium to an extracellular acceptor appears to be common in nature. The electron acceptor can be another cell or abiotic material. Such extracellular electron transfer contributes to syntrophic metabolism and is of wide environmental, industrial, and medical importance. Electron transfer between microorganisms and electrodes is fundamental in microbial fuel cells for energy production and for electricity-driven synthesis of chemical compounds in cells. In contrast to the much-studied extracellular electron transfer mediated by cell surface exposed cytochromes, little is known about components and mechanisms for such electron transfer in organisms without these cytochromes and in Gram-positive bacteria such as E. faecalis, which is a commensal gut lactic acid bacterium and opportunistic pathogen.

Electroactivity across the cell wall of Gram-positive bacteria

“…Electroactive organisms can be found in all three domains of life, being ubiquitous in distinct environments, including lakes, soils as well as in deep-sea hydrothermal vents [11] , [12] . Recently, is has been demonstrated that these organisms are also present in the human digestive system [16] , [17] , in the mouse gut microbiome [18] , [19] , [20] and oral plaque [21] , with some of them associated with infectious diseases [22] , [23] .…”

“…Gram-negative mesophilic bacteria are one of the most studied class of electroactive organisms, with most of the knowledge being confined to the model organisms Geobacter sulfurreducens and Shewanella oneidensis MR-1 [2] , [3] , [24] . Nonetheless, Gram-positive bacteria have recently attracted the scientific attention, given their capacity in producing high levels of current in MFC [25] , [26] , and by being associated with infectious diseases in humans [16] , [17] , [22] , [23] . Given the importance of these organisms in BES, research has been dedicated in exploring their use as catalysts in BES and in the understanding of their EET processes [27] , [28] , [29] .…”

Extracellular Electron Transfer: Respiratory or Nutrient Homeostasis?

“…The paper from Hederstedt, Gorton, and Pankratova in this issue of the Journal of Bacteriology (22) uniquely approaches this question by directly comparing ferric iron reduction and EET abilities of E. faecalis mutants. Naturally, ferric iron reduction is an extracellular reaction and thus requires EET.…”

“…Because one of the E. faecalis NDH-2s (Ndh2) is highly homologous to L. monocytogenes NDH-2, it is likely that this E. faecalis NDH-2 also catalyzes NADH:DMK oxidoreduction and is thus a "conventional" NDH-2. Indeed, Hederstedt et al showed that, using a membrane extract, NADH oxidation activity was associated with NDH-2 (22). Then, the question remains, how do E. faecalis and L. monocytogenes (and other Gram-positive bacteria) utilize multiple NDH-2 enzymes to generate distinctive electron transfer pathways?…”

“…The biochemical and biological function of this extension is currently unknown and difficult to deduce from their amino acid sequences, given that this extension is diverse among Eet NDH-2 enzymes from different organisms (21). However, as pointed out by Hederstedt et al, E. faecalis and L. monocytogenes Eet NDH-2 have highly homologous C-terminal extensions (22). The Eet NDH-2 enzymes from E. faecalis and L. monocytogenes might thus form a specific electron transfer protein complex with other proteins to efficiently oxidize quinone and release electrons to extracellular acceptors.…”