Flavin-Cytochrome Complexes Transfer Electrons among Shewanella oneidensis MR-1 Cells

The metabolic activity of microorganisms in multicellular assemblages is limited by structural constraints and resource availability. To overcome these limitations, some bacteria utilize electron conduction in their communities to drive their metabolism, which has led to the development of various biotechnologies, such as electrochemical microbial systems and anaerobic digestion. However, measuring the conductivity among bacterial cells is difficult when they scarcely form stable biofilms on electrodes, which renders it difficult to identify the biomolecules involved in electron conduction. In the present study, we aimed to identify the proteins involved in electron conduction in Shewanella oneidensis MR-1 and examine the molecular mechanisms. We established a colony-based current–voltage measurement system that quantifies bacterial electrical conductivity, without the need for biofilm formation on electrodes. This assay enabled the quantification of the conductivity of gene deletion mutants that scarcely form biofilms on electrodes, demonstrating that c-type cytochromes, MtrC and OmcA, are involved in electron conduction. Furthermore, the use of colonies of gene deletion mutants demonstrated that flavins participate in electron conduction by binding to cytochromes on their outer membrane, providing insight into the electron conduction pathways at the molecular level. Furthermore, the conductivity of Bacillus subtilis 3610 colonies was determined to be approximately 23 times lower than that of MR-1 colonies, indicating that this approach can be used for various bacteria, including weak electricigens. The present assay provides a platform for rapidly identifying conductive bacterial colonies and components, linking bacterial energy conservation with electron transfer in multicellular communities.