Anaerobic incubation of E. coil K12 strains in an electrochemical cell induced the generation of an electrical current under constant polarization at 0.52 V/SHE. After incubation under potentiostatic conditions, the cyclic voltammetry signature points towards deposits of an insoluble layer of an oxidized redox compound. This bacterial redox mediator is soluble in its reduced state and exhibits a standard redox potential of 0.03 V/SHE at pH 7 with a slope of 120 mV per pH unit. A thorough inspection of the cyclic voltammetry allows us to shed light on how the bacteria interact with the electrode. Use of mutant strains of E. coil K12 in which genes of the synthetic pathways of ubiquinone and menaquinone were deleted did not interfere with the secretion of the redox mediator. (C) 2014 Elsevier Ltd. All rights reserved.