Abstract
Gram-negative Escherichia coli strain JM109 looks promising for energy generation in biofuel cells. The cells we are using over express the di-heme membrane protein from Bacillus subtilis succinate:quinone reductase. This protein has two b-type hemes: heme bL with a midpoint potential of -132 mV vs NHE and heme bH with a midpoint potential of+16 mV vs NHE. The protein is present in the inner cell membrane and can potentially be used to enhance the wiring between the cell and different electron transfer mediators. The role of mediators in microbial biosensors and biofuel cells is to take electrons from the oxidizing enzymes and/or respiratory chain and transport them to the surface of the electrode and thus to replace the natural electron acceptor - oxygen, in the case of aerobic bacteria for example. In this way artificial electron shuttles can enhance electron transfer between microbial cells and electrodes. It is well known that different water-soluble mediators such as 2,6-dichlorophenol indophenol (DCPIP) or ferricyanide (FeCN) and hydrophobic ferrocene derivatives can be effectively used for coupling microbial metabolism and electrodes . One recent trend in wiring of living cells and electrodes is to use polymeric mediators, which exhibit efficient electron shuttling properties for multiple layers of microbial cells [2,3]. In the current study the application of different artificial electron transport mediators for efficient electrical wiring of gram-negative bacteria E. coli JM109 were investigated. Different characteristics of the system containing a graphite electrode modified with cells, a Ag|AgCl (0.1 M KCl) electrode and a platinum wire used as reference and auxiliary electrodes, respectively, were evaluated in flow analysis mode when glucose was used as a substrate. The influence of pH of buffer solution and presence of oxygen on the current response was evaluated. E. coli JM109 cells immobilized on the surface of graphite electrode did not show any direct electron transport. We have not been able to show any current response from the cells in the presence of DCPIP-solution or ferrocene immobilized on the surface of the electrode. However we obtained very good results with a flexible osmium redox polymeric mediator and the water-soluble mediator - ferricyanide.