Giving bacteria new and useful functions
By choosing appropriate genes to introduce into bacteria, scientists have developed bacteria with characteristics that are useful to humans – such as the ability to make important compounds (such as fuels) or to break down noxious ones (such as environmental contaminants).
How to give a bacterium new characteristics
This is a routine lab procedure that usually includes the following steps:
Choose a bacterium to modify. Depending on the new function you’re interested in, you might choose a disease-causing bacterium, a photosynthetic bacterium or the model bacterium E. coli.
Identify a protein with a potentially useful function. This might be an enzyme that carries out an industrially important reaction or a protein that fluoresces in certain conditions.
Clone the gene that encodes the protein into an expression vector that contains an appropriate promoter.
Introduce the expression vector containing ‘your’ gene into the bacterium that you wish to modify.
Grow the modified bacteria.
Test whether the bacteria are expressing the introduced protein and whether they have acquired new characteristics.
Find out more about How to add foreign DNA to bacteria and Producing foreign proteins in bacteria.
Glowing Mycobacterium in mice
Siouxsie Wiles (University of Auckland) and her team have modified Mycobacterium tuberculosis (which causes TB) to produce light. The scientists treat mice with the modified bacteria, then track the course of infection by measuring how much light is emitted from the mouse. These mice (M1–M4) have living Mycobacterium in their lungs. Note that the image has been artificially coloured to show how much light is emitted (red is the strongest light emission and blue is the weakest).
Examples of modified bacteria
Bacteria that can perform useful functions are being produced around the world. The three examples given below are from New Zealand university labs and are intended to show how diverse the applications of modified bacteria can be. It’s important to note that all the bacteria described are in use within a laboratory context only, for research purposes. They aren’t being released into the environment.
Making infectious bacteria that glow
Siouxsie Wiles (University of Auckland) and her colleagues study Mycobacterium tuberculosis, the bacterium that causes the lung disease tuberculosis (TB). They have modified M. tuberculosis using genes that encode proteins that make light. These genes come from organisms that ‘glow’, such as fireflies. The result is modified bacteria that emit light at a particular wavelength, which can then be detected and measured (even when the bacteria are inside mice).
By using the modified bacteria, Siouxsie and her team can quickly test potential new drugs for treating TB. They do this by infecting mice with the bioluminescent M. tuberculosis and then treating the mice with a possible drug to see if it kills the bacteria. As the modified M. tuberculosis glows only when it’s alive, any drug that is effective in killing the TB bacteria results in a decrease in bioluminescence.
Siouxsie’s approach is an improvement on other methods of testing anti-TB drugs, as these all rely on the mice becoming visibly affected by TB. With bioluminescent bacteria, the progress of the infection can be monitored in real time, so the effect of the drug can be observed earlier, and the experiment is stopped much sooner. It takes less time, uses fewer mice and is more humane.
Listen and find out more in this RNZ audio, Bioluminescence and superbugs.
Making bacteria that produce fuel
Julian Eaton-Rye and Ryan Hill (University of Otago) study Synechocystis sp. PCC 6803. This is a bacterium that – like plants – uses the energy of sunlight to produce sugars (the process of photosynthesis). By introducing genes that encode metabolic enzymes into Synechocystis, Julian and Ryan have given the bacteria the ability to convert these sugars into butanol.
Growing Synechocystis
Julian Eaton-Rye studies a flask in which the bacterium Synechocystis sp. PCC 6803 is growing. The culture is green because the bacteria contain chlorophyll – the green pigment that harvests light during photosynthesis. Scientists in Julian’s laboratory have introduced genes to Synechocystis that give it the ability to convert sugars into butanol.
As well as being an industrially important chemical, butanol can be used as fuel in cars in place of petrol, so any system for producing butanol in large quantities that didn’t rely on fossil fuels could be important for meeting our global energy needs in the future.
Making bacteria that produce tools for bioremediation
David Ackerley (Victoria University) and his team have been exploring ways to detoxify hexavalent chromium. This serious environmental pollutant is a byproduct of numerous industrial processes and is a dangerous carcinogen. Dave and his colleagues have shown that an E. coli enzyme known as NemA can transform hexavalent chromium into chromium(III), which is relatively non-toxic.
Working with Bernd Rehm (Massey University) and colleagues, the scientists cloned the gene encoding the NemA enzyme alongside another gene from the bacterium Ralstonia eutropha. The second gene encoded an enzyme that makes a form of bioplastic. They then expressed the two genes as a single protein in E. coli. The result? Bioplastic ‘beads’ that had NemA displayed on their surface.
The NemA bioplastic beads can be easily purified away from the bacteria they were made in, then used separately to detoxify hexavalent chromium. This way, the modified E. coli are producing a new detoxifying compound but are not themselves being released into the environment.
Useful links
Watch this short animation by Siouxsie Wiles to learn more about her TB research with bioluminescent bacteria.
Read about David Ackerley’s earlier research into detoxifying hexavalent chromium (and his experiences as a postdoctoral fellow in California) in this article in the New Zealand Science Review (pp 70–73).
