Likely mechanisms of direct and indirect electron transfer processes have been identified from studies of dissimilatory metal reducing bacteria that use insoluble iron oxides as a terminal electron acceptor . Three main pathways for electron transfer have been identified and include: the production of electron shuttles , direct contact of outer surface c-type cytochromes and long range contact via electrical conductive pili or nanowires (Figure 3). The advantages of direct electron transfer and the use of pili to form a conductive biofilm has over the production of shuttles has been extensively reviewed elsewhere