BACKGROUND Many approaches have been employed to increase the understanding and consequently the performance of microbial fuel cells to obtain simultaneous power production and biodegradation. This study uses recombinant Escherichia coli K-12 with MtrA, MtrC and MtrCAB inserts previously prepared using synthetic biology to evaluate the involvement of each of these genes in bioenergy production and biodegradation of Congo red using a double chamber microbial fuel cell. RESULTS MtrC was the key gene required for energy production corresponding to an average voltage of 360 mV (external resistance 1 k) and power density of 59 mW m(-2), while E. coli with MtrCAB insert showed the highest decolourisation which reached 80% in 36 h under microbial fuel cell conditions. Coulombic efficiency was 1.2% for E. coli with MtrCAB compared with 2.5% and 2.3% for MtrC and MtrA inserts, respectively. Riboflavin seems to be involved in the electron transferring, its concentration was highest for E. coli with MtrA insert despite its poor performance for both bioenergy production and dye degradation. CONCLUSION This study suggests that electrons are mutually exclusive between electricity production, dye degradation and other cellular activities. This study helps to improve our understanding of the dual bioenergy/decolourisation process taking place in MFCs in order to maximize the outcome. (c) 2018 Society of Chemical Industry