Friday, August 23, 2013

E. coli Biosensors: Going for the Gold

All that glitters is not gold, and the shine of most modern gold deposits are hidden underneath layers of dirt, soil and sand. Finding these deposits usually requires expensive and time-consuming chemical analysis of soil samples. Recently, an international team of reseachers met this challenge of gold exploration and prospecting by turning a common gut microbe, Escherichia coli (E. coli), into a miniature gold detection device.

In a recent paper published in PLoS One, researchers from the University of Nebraska and their collaborators in Australia detail how they have genetically modified E. coli to act as a gold biosensor by borrowing the golTSB genes from a closely related microbe, Salmonella typhimurium. By pairing these gold recognition genes to a known enzymatic activity, researchers can detect and quantify small amounts of gold by simply measuring a change in the color of the bacteria-containing solution. The gold detection limit for this biosensor is on par with that of the chemical analysis currently used in the industry, which is slower and involves much more expensive instruments.

These proof-of-concept studies, which were partly funded by both Newmont Exploration Proprietary Limited and Barrick Gold of Australia Limited, are the latest step towards the development of a quick, accurate and specific biosensor that will make examining potential gold mining sites easier and faster. The authors of the study demonstrate that their biosensor can be used to determine the concentration of gold in a soil sample or a sample containing multiple metals. This is an improvement over earlier research of prototype biosensors, which only demonstrated detection in relatively pure samples.

For more detail and commentary about this study, please select 'Read More'. Do you think cell based biosensors will revolutionize gold exploration? Comments are welcome below!

From bacterial stress to mining success

In order to cope with the stress imposed by toxic heavy metals, gold included, most bacteria have evolved dedicated proteins to recognize and export these metals from the cell. While many bacteria have that proteins recognize and respond to all toxic metals indiscriminately, the Salmonella golTSB system displays a relatively rare specificity to gold. This system is made up of three genes, the most salient being the regulatory protein GolS. Under normal conditions, the GolS protein detects gold and responds to its presence by activating the other genes in the golTSB system. Previous research has demonstrated that this response is specific to gold, and not responsive to other metals such as copper, zinc, cobalt, iron, lead and nickel.

To create a gold detecting biosensor, researchers placed the lacZ gene, which produces the enzyme Beta-galactosidase, under the control of the GolS protein and transplanted the entire system into the more easily manipulated bacteria E. coli. The presence of greater amounts of gold will lead GolS to more strongly activate lacZ and produce increased amounts of Beta-galactosidase. This enzyme can cleave a dye off of a modified sugar molecule, changing the color of the cells or solution in which it resides. The change in color then can be detected either using common laboratory instruments or even, under the right circumstances, with the naked eye. Most importantly, the degree of the color change is proportional to the amount of Beta-galactosidase produced, allowing researchers to use the color change to quantify the amount of gold in a solution.

The E. coli biosensor was able to detect and quantify gold in a solution, and discriminate between gold and several other metals when mixed together (excluding copper, which give a considerable false positive signal). The researchers also tested the ability of the biosensor to detect known amounts of gold in spiked soil samples. The soil samples were taken from diverse geographical locations in Australia and had very different compositions and characteristics. Nonetheless, after standard treatment of the soil with thiosulfate to ensure all the gold was solublized and detectable by the biosensor, the presence of gold in these samples was confirmed.

Biosensors as a golden ticket

Gold is a very important precious metal, serving both as a commodity, security, and building material in different industries. Identification of new gold deposits is a major commercial interest, but most easily identified deposits have already been found and mined. Biosensors such as the one featured in the PLoS One study may have a significant impact on how quickly and economically new deposits are found.

The E. coli biosensor developed by the international team of US and Australian researchers, as well as a similar biosensor developed by an Argentinian team, represent important steps towards revolutionizing gold prospecting. However, further refinement of this technology is necessary. First, more rigorous determination of the biosensor capabilities is required. While this study is an improvement over previous research because more realistic samples were used (soil samples and metal mixtures), a larger range of gold concentrations should be tested. Also, the effect of gold and other metals on the physiology of the bacteria needs to be better characterized. As mentioned in the study, the toxic effects of high concentrations of metals can compromise the ability of the bacteria to express the reporter. Finally, the pivotal experiment in which real soil samples were used lacks an important control: the unspiked soil (not containing gold). It is critical that these experiments are repeated with proper controls before biosensor technology can be brought from the lab bench to a prospective gold mine.

Further characterization and optimization of gold biosensor technology, perhaps with novel strategies using different reporters or even different organisms (a plant-based biosensor that reports via flower-color would certainly have advantages), has the potential to greatly accelerate gold prospecting efforts. In addition to bioengineering bacteria to harvest gold, these efforts pave the way for similar advancements in many other industries. In some sense, we may be standing upon the edge of a golden age in biotechnology!

Selected References and Links

Zammit, et al (2013) A whole-cell biosensor for the detection of gold PLoS One Vol 9 (8) PMID: 23950889
This is study that is featured in this 'press release' or summary.

Cerminati, et al (2011) Selective detection of gold using genetically engineered bacterial reporters. Biotechnology and Bioengineering Vol. 108 (11) PMID: 21618467

A similar study by a group in Argentina, which utilized the GFP gene as a reporter instead of B-galactosidase. While this study did not demonstrate the ability of the GFP biosensor to detect gold among different ions or in a complex soil sample, this is for lack of trying (the experiment was never performed). So it remains possible that the GFP reporter is as good as or better than the B-gal biosensor.

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