Electrons: A new growth media for bacteria

Dr. Neil Canter, Contributing Editor | TLT Tech Beat May 2013

A process known as electrochemical cultivation enables a specific bacterium to use electrons as the sole energy source.

KEY CONCEPTS
• Bacteria that oxidize Fe (II) to grow can cause corrosion mainly in marine applications such as bridges, piers and ships.
• A technique known as electrochemical cultivation has been used to enable these bacteria to use electrons as their sole energy source.
• This work removes a bottleneck that has prevented the use of microbes from synthesizing organic compounds that would include fuels.

IN THE SEARCH FOR ALTERNATIVE ENERGY SOURCES, more attention is being paid to looking at how microbes obtain energy to meet their needs as growing organisms. Their ability to adapt to their environment and utilize different strategies is leading to a variety of different applications.

For example, a previous TLT article highlighted how a specific bacterium can be modified to be used as a sensor by taking advantage of a metal-reduction pathway that it uses for respiration (1). The bacterium Shewanella oneidensis MR-1 uses a specific membrane protein, MtrB in this pathway. By modifying the pathway so that the bacterium only respires in the presence of a specific compound, the researchers were able to detect arsenic down to a 10 ppm level in water used in processing oil sand.

Another interesting type of bacteria that is challenging to study is microbes that oxidize ferrous [Fe (II)] ions. Dr. Zarath Summers, a postdoctoral researcher working in the laboratories of Dr. Daniel Bond and Jeffrey Gralnick at the University of Minnesota in Saint Paul, Minn., says, “Fe (II)-oxidizing bacteria play a pivotal role in the iron cycle that involves conversions between ferrous and ferric ions. They act in coordination with iron reducers, which are found in hydrothermal vents and acid mine drainage.”

Summers continues, “Fe (II)-oxidizing bacteria respire oxygen at low concentrations because higher levels of oxygen will facilitate iron oxidation and interfere with their growth. Working with this type of bacteria is very difficult because they are hard to grow. Imagine a culture of these bacteria oxidizing iron, leading to a bright orange, turbid medium in which it is difficult to see the microbes.”

Summers estimates that these bacteria require 120 moles of iron to make 1 mole of glucose. She adds, “It is a tough way to make a living.” It is difficult to do any biochemistry on these cells, which includes isolating and purifying proteins.

Fe (II)-oxidizing bacteria instigate corrosion mainly in marine applications such as bridges, piers and ships, according to Summers. They are not likely to be involved in fuel pipelines where sulfate-reducing bacteria are the main culprits.

Summers and her fellow researchers have been able to prompt other bacteria to generate current. They figure that with iron-oxidizing bacteria, there is an opportunity to, in effect, go the other way by utilizing electrons for growth. Such a process has now been developed.

ELECTROCHEMICAL CULTIVATION
Working with the bacterium, Mariprofundus ferrooxydans strain PV-1, the researchers have used a technique known as electrochemical cultivation to use electrons as the sole energy source. The bacteria were introduced into 80 ml, three-electrode reactors containing polished graphite electrodes that were maintained at a potential of -0.076 V versus the standard hydrogen electrode. The cells were continuously flushed with a mixture of oxygen, carbon dioxide and nitrogen maintained at a ratio of 8:10:82.

Summers says, “We chose a higher potential to steer clear of any possibility of water reacting to generate hydrogen, which could have been used by the bacteria as a significant electron donor. Our intent was to have low oxygen, neutral pH conditions that mimic the natural conditions and would facilitate the consumption of electrons by the bacteria.”

The researchers found that upon addition of the bacteria to the three-electrode reactor, a steady pickup of electrons was detected and ultimately reached a plateau after four weeks of growth. Replacement of the medium in the first setup and inoculation of a second series of electrochemical cells did not affect the process. The bacteria multiplied and formed a distinct film on the electrode.

Figure 2 shows a scanning electron micrograph of the bacteria attached to the graphite electrode surface.


Figure 2. Using a technique known as electrochemical cultivation, Fe (II)-oxidizing bacteria started to use electrons as their sole energy source by attaching themselves to a graphite electrode surface, as shown in this scanning electron micrograph. (Courtesy of Zarath Summers/University of Minnesota)

Summers says, “We found that the bacteria kept pumping along for up to a period of six months without any problems. No negative byproducts were detected during this time.” One other factor was that bacterial growth took place in the absence of any iron.

The only things that stopped the process were the absence of oxygen and the electrochemical cell. Transfer of electrodes into a sterile medium containing iron metal plates led directly to the formation of orange rust characteristic of ferric iron oxides.

Future work will involve trying to determine the mechanism for the uptake of electrons and of Fe (II) ions by the bacteria. Summers says, “We suspect that the process involves one or more extracellular components that are electron transfer proteins.”

This work removes a major bottleneck preventing microbes from synthesizing organic compounds that would include fuels. One important aspect is that the energy obtained by the organism can be used to reduce carbon dioxide so that it becomes a source for organic compounds.

Summers says, “We believe that the bacteria can be genetically modified to make the cell produce any type of organic compound that is requested. In addition, the genes from this bacterium can be removed and placed in another species to facilitate production of other compounds.”

Further information can be found in a recently published article (2) or by contacting Summers at zsummers@umn.edu.

REFERENCES
1. Canter, N. (2013), “Bacteria-Containing Biosensor,” TLT, 69 (1), pp. 14-15.
2. Summers, Z., Gralnick, J. and Bond D. (2013), “Cultivation of an Obligate Fe(II)-Oxidizing Lithoautotrophic Bacterium Using Electrodes,” mBio, 4 (1), doi:10.1128/mBio.00420-12.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.