Cyclic voltammetry (CV) and electrochemical in situ FTIR spectroscopy (in situ FTIRS) were used to investigate the directly extracellular electron transfer (EET) of Shewanella oneidensis MR-1 wild type and Delta omcA-Delta mtrC mutant in this work. The CV results illustrate that the mutant still possesses electron transfer capability, but much weaker than the wild type does. In this in situ FTIRS study, some new IR bands ascribing to OmcA-MtrC protein and CO2 were firstly collected. The EET process can be evaluated through monitoring the intensity of these bands. In water solution, the wild type has a band at 1742 cm(-1) of nu((C = O)) while the mutant has one at 1712 cm(-1) of nu((C = O)) during EET process. However, the band at 1742 cm(-1) disappears but the band at 1712 cm(-1) appears when wild type is in deuterated water solution; it is noteworthy that 1742 cm(-1) can reappear after the sodium lactate was added into the solution. For Delta omcA-Delta mtrC mutant, the 1712 cm(-1) band is present in both water and deuterated water solution. These all suggest that the band at 1742 cm(-1) is ascribed to the OmcA-MtrC protein while the band at 1712 cm(-1) belongs to some unknown protein. Meanwhile, we have also found that the band at 2342 cm(-1) is contributed by CO2 produced by the bacterial metabolism. The CO2 band in the wild type was larger than that in the mutant whether the two bacteria were fed lactate or not, the data implied that electron transfer capability of the wild type is stronger than that of the mutant. We thus proposed that the amount of CO2 can serve as a key index to evaluate the EET capability. Electrochemical in situ FTIRS is much stronger in quantitatively explanation why the wild type has a greater electron transfer capacity compared with the mutant; the function of OmcA-MtrC protein in EET process can be extracted during cell respiration. (C) 2015 Elsevier Ltd. All rights reserved.