Background: Extracellular electron transfer (EET) is the key feature of some bacteria, such as Geobacter sulfurreducens and Shewanella oneidensis.Via EET processes, these bacteria can grow on electrode surfaces and make current output of microbial fuel cells.C-type cytochromes can be used as carriers to transfer electrons, which play an important role in EET processes.Typically, from inner membrane through periplasm to outer membrane, they could form EET pathways.