The direct electron-transfer (DET) property of Shewanella bacteria has not been resolved in detail due to the complexity of in vivo electrochemistry in whole-cell systems. Here, we report the in vivo assignment of the redox signal indicative of the DET property in biofilms of Shewanella oneidensis MR-1 by cyclic voltammetry (CV) with a series of mutants and a chemical marking technique. The CV measurements of monolayer biofilms formed by deletion mutants of c-type cytochromes (Delta mtrA, Delta mtrB, Delta mtrC/Delta omcA, and Delta cymA), and pilin (Delta pilD), capsular polysaccharide (Delta SO3177) and menaquinone (Delta menD) biosynthetic proteins demonstrated that the electrochemical redox signal with a midpoint potential at 50 mV (vs. SHE) was clue to an outer-membrane-bound OmcA-MtrCAB protein complex of decaheme cytochromes, and did not involve either inner-membrane-bound CymA protein or secreted menaquinone. Using the specific binding affinity of nitric monoxide for the heme groups of c-type cytochromes, we further confirmed this conclusion. The heterogeneous standard rate constant for the DET process was estimated to be 300 +/- 10s(-1), which was two orders of magnitude higher than that previously reported for the electron shuttling process via riboflavin. Experiments using a mutant unable to produce capsular polysaccharide (Delta SO3177) revealed that the DET property of the OmcA-MtrCAB complex was not influenced by insulating and hydrophilic extracellular polysaccharide. Accordingly, under physiological conditions, S. oneidensis MR-1 utilizes a high density of outer-membrane-bound OmcA-MtrCAB complexes as terminal reductases for the DET electrode-respiring process. (C) 2011 Elsevier Ltd. All rights reserved.