Extracellular electron transfer features of the Gram-positive bacterium Enterococcus faecalis

Sammanfattning: Electroactive microorganisms possess the unique ability to transfer electrons to or from a solid phase electron conductor, e.g., electrodes or minerals, through various physiological mechanisms. The processes are commonly known as extracellular electron transfer and broadly harnessed in microbial electrochemical systems, such as microbial electrosynthesis or microbial fuel cells. Except for a few model microorganisms, the nature of the microbe-electrode conductive interaction is poorly understood for most of the electroactive species, which determines the efficiency and the potential scaling up of bioelectrochemical systems. The use of electron transfer mediators, such as electron-conducting polymers, is considered as one of the promising strategies to enhance the electron transfer efficiency up to the scale of a real life application.Gram-positive bacteria are microorganisms carrying a thick electron non-conductive cell wall and believed to exhibit weak extracellular electron shuttling activity. Most of the reported cases are barely understood and offer poor and frivolous knowledge about the electron transfer and associated mechanisms in Gram-positive bacteria. Far less is known about the electron transfer of the bacteria mediated by high molecular weight redox polymers. This thesis presents studies on the extracellular electron transfer mechanisms and properties of the Gram-positive bacterium Enterococcus faecalis. The bacterium was confirmed to perform extracellular electron transfer both directly and via mediators. The wild-type strain and some mutants were investigated electrochemically in combination with various mediating compounds. The E. faecalis cells are demonstrated to transfer electrons to electrode surfaces via a demethylmenaquinone pool in the membrane. Heme proteins presented in the cells under aerobic conditions are shown to hinder such electron transfer processes.Furthermore, the cell-polymer-electrode mediated interaction through osmium and quinone-based polymeric mediators is characterized in detail. The obtained experimental data suggest that a redox polymer can be directly incorporated into the respiratory chain of the bacterial cells for accepting electrons and to further shuttle them to the electrode surface. The attained results will help in further development and adaptation of mediators for microbial-based systems.The findings may add an essential piece of fundamental understanding of the nature and possibilities of microbial extracellular electron transfer, and advance our knowledge in interspecies electron transfer and the cycling of biogeochemical elements in nature. Additionally, a comprehensive understanding of cell-electrode interactions may help in overcoming insufficient electron transfer and restricted operational performance of various bioelectrochemical systems.