Shewanella oneidensis MR-1 is an important microorganism in bioremediation due to its diverse respiratory capabilities. It is a dissmilatory metal reducing bacterium. Being facultative organism, it has the ability to adapt to both aerobic and anaerobic environments, as well as utilise a number of toxic compounds such as manganese and uranium. It does this by accepting electrons, which gives Shewanella oneidensis the potential of producing electricity. An electron transfer pathway, known as the Mtr pathway, facilitates the extracellular translocation of electrons that are gained during the reduction of a carbon source.
The Mtr pathway consists of the following five genes:
We have been working on introducing the Mtr pathway into Escherichia coli, in order to increase the electrical output in a microbial fuel cell (MFC). Shewanella oneidensis is capable of transferring these electrons through extensions known as nanowires. However, E.col have a far greater genetic tool set at their disposal, enabling easy genetic manipulation and modification. Further more, they are a very robust microorganism that, when compared to Shewanella oneidensis, are better equipped to deal with changes in a range of environmental variables, e.g. temperature and pH. The carbon source for our modified bacteria will be wastewater.
We are exploring the possibility of electron transfer through the use of flagella found in E.coli K-12 derivative, DH5-α. E.coli should act as an anode. Formation of biofilm has also been noted in this strain of E.coli. The efficiency of the MFC is down to the biofilm, which is formed when cells adhere to a surface and stick to each other. The composition of these biofilms is what determines the amount of electricity produced as it acts like a conductive matrix, enabling higher kinetics rates of electron transfer along nanowires.
This technology has a multitude of application. It could be used as an electrical source for hard to reach biosensors such as a deep-water biosensors within a sediment microbial fuel cell. Furthermore, MFSc could play a major role in bioremediation, which is removal of organic pollutants from environments to produce clean water and excess electrical energy.
What is Microbial Fuel Cell?
MFC (Figure 1) is a device that utilises micro-organisms ability to catalyse an oxidation and reduction reaction at an anode and cathode electrode, respectively, and can produce electricity when connected to a load/resistor via an external circuit, whilst producing water at the cathode.
How microbial fuel cell (MFC) works
In anaerobic conditions, when no oxygen is present the microorganisms produce carbon dioxide, protons and electrons when consuming a carbon source. The architecture of a MFC capitalises on this principal to create electricity. Once the bacteria form a biofilm around the anode, and start to break down organic matter, electrons are striped from the carbon source by an oxidation reaction and translocated out of the cell via an electron transport pathways found only in these specific exoelectrogenic bacteria. These donated electrons are transferred to a carbon electrode (anode) and moved along a wire to a cathode. As electrons move through the wire, they pass through a resistor and an electrical current is generated. At the cathode the electrons reduce atmospheric oxygen to clean water with the addition of protons that are generated during the oxidation reaction.
Figure 1: Animated working model of a microbial fuel cell
Adapted from https://www.youtube.com/watch?v=DVI6tMP-rOY/