The 1950s were a crazy time for science. The Cold War had nuclear weapons and radioactive fallout on everyone’s mind, and Arthur W. Anderson was no exception. In 1956, Anderson experimented with the possibilities of sterilizing canned foods through gamma ray bombardment. After exposing tinned meat to doses of radiation thought to kill all known forms of life, however, he found that one bacterium remained standing: the red, spherical Deinococcus radiodurans.
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| Figure 1. D. radiodurans under light microscopy. |
With a name like “strange berry that withstands radiation,” it makes sense that D. radiodurans was able to last in the radioactive conditions of Anderson’s canned foods. But most organisms can’t last in these kinds of conditions. Unlike most organisms, D. radiodurans has a unique DNA repair system that gives it the ability to of withstand high levels of radiation. High radiation doses cause the D. radiodurans genome (or, the full set of genes in an organism) to shatter. However, unlike other organisms, D. radiodurans is capable of quickly stitching the shattered fragments of its genomes back together - up to 500 fragments at a time, in fact. Furthermore, the D. radiodurans genome carries four to ten copies of its genome rather than one copy, as most organisms do. These additional genomes are thought to allow the microbe to recover at least one full copy of its genome after radiation exposure, thereby facilitating the proteins and pathways involved in stitching the shattered genomes back together.
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| Figure 2. D. radiodurans model for repairing its genome after being shattered. |
Excluding D. radiodurans, most bacteria die when exposed to sixty grays (a unit of measurement that describes absorbed radiation) of radiation. Humans succumb to the effects of radiation at about ten grays. Meanwhile, D. radiodurans can survive an exposure of 15,000 grays. Even when exposed to chronic levels of radiation, D. radiodurans can resist far more radiation than any other organism. In fact, the levels of radiation that D. radiodurans can survive do not exist naturally on Earth and have only been simulated in labs. D. radiodurans is able to thrive and continue to survive in highly irradiated environments thanks again to its robust DNA repair system!
But even though this cool repair system is so well known, it isn’t known why D. radiodurans evolved such a complicated DNA system of repairing radiation damage in the first place. This is because no environment on Earth exposes life to even a fraction of the levels of radiation that D. radiodurans can withstand. However, it turns out that dehydration and radiation can cause similar types of DNA damage. After all, D. radiodurans can withstand more than just radioactive conditions. It can also survive drought, extreme cold, vacuums, high acidity. and lack of nutrients. The unique DNA reparation system is again why D. radiodurans can withstand all tough environments, not just tough radioactive environments. As such, it has since come to be known as “the world’s toughest bacterium” according to institutions like the Guinness Book of World Records. This is why D. radiodurans is frequently referred to as the “Conan the Barbarian” of the microbial world!
All of these characteristics have made D. radiodurans an ecological and environmental pioneer that has traversed and survived some of the most perilous living environments on Earth. Organisms that can withstand environments that would otherwise be uninhabitable are called extremophiles. As the name suggests, extremophiles persist in habitats that lie on the far ends of various environmental spectrums that inhibit typical organisms from living and reproducing. Other extremophiles have been found in the subzero temperatures in the Arctic, or in the boiling waters of hydrothermal vents deep in the ocean. While extremely cold, hot, acidic, or salty surroundings usually inhibit growth and damage DNA to the point of preventing replication, extremophiles have developed unique mechanisms to cope with their challenging lifestyle. Some organisms hunker down to survive the cold, while others have specialized proteins or internal mechanisms that respond to changes in pH or salinity.
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| Figure 3. The extremophilic nature of D. radiodurans when faced with tough environments. |
But once again, while D. radiodurans joins the ranks of fellow extremophiles that can endure the excessive cold or exceptionally dry conditions, the area where D. radiodurans shines is in radioactive locales. And because of the DNA repair capacity and extremophilic nature of D. radiodurans, D. radiodurans is increasingly being used as a host organism for bioremediation. Bioremediation is any process in which microorganisms, fungi, or plants are used to return an environment to its natural condition after being altered by contaminants like radionuclides, heavy metals, and toxic solvents. However, bioremediation of radioactive environments is difficult because the number of useful microorganisms are limited due to radiation tolerance. As such, D. radiodurans is ideal in treating radioactive waste and has been genetically engineered accordingly to best consume and digest contaminants in radioactive environments. For instance, researchers have engineered D. radiodurans to detoxify ionic mercury residue commonly found in radioactive waste from nuclear weapons manufacture by incorporating the mercuric reductase gene (a gene capable of stabilizing mercury) from Escherichia coli, which cannot handle high radiation levels like D. radiodurans. This makes D. radiodurans less of a barbarian, and more of a doctor or clean-up crew for the environment!
Bioremediation is important because radioactive wastes are produced by more than just nuclear plants - even hospitals produce radioactive wastes. So it’s important that we come up with a mechanism like bioremediation that will allow us to treat contaminated areas without excavating them, which risks more exposure to radiation. For instance, the nuclear accidents at Three Mile Island (1979), Chernobyl (1986), and Fukushima (2011) occurred prior to much of our knowledge about bioremediation, and resulted in extensive environmental and health damages because they had to be excavated to treat. This serves to make bioremediation a more cost-effective method of managing polluted environments since it is less invasive. Overall, bioremediation is important because severe environmental and public health crises can result when contamination is allowed to enter the atmosphere, groundwater, and seawater.
One of the most hazardous elements resulting from nuclear plant accidents is radioactive iodine. However, it is also one of the few radioactive elements that D. radiodurans has not been used to bioremediate. This is interesting because radioactive iodine waste has had an extensive history in both nuclear plants and hospitals - two areas where radioactive waste production is common. For instance, radioactive iodine was found to have been the most common metal radionuclide released by Fukushima. In addition, radioactive iodine has been extensively used in biomedical applications such as in adjuvant therapy for thyroid cancer patients, but has been used without proper disposal, instead being abandoned directly into the environment.
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| Figure 4. A map of radioactive iodine released by Fukushima. |
But the question then becomes: how can D. radiodurans be used to bioremediate radioactive iodine? The answer is a little tricky, and involves a material you would never think to combine with bacteria - gold! Recently, research has shown that gold may be as valuable to biomedical researchers as it is to the rest of us. Because of its stability and ability to remain inert, the biotechnology field has been observing gold with growing interest. Gold has been used in novel imaging technology, and could soon be used as a courier for anti-cancer drugs. Because these applications are microscopic in nature, the gold used must also be reduced to a very small and uniform size. These very small amounts of gold are called gold nanoparticles.
One recent study has shown that bacteria like D. radiodurans are capable of transforming large gold particles into nanoparticles. When a solution containing liquid gold is combined with D. radiodurans, the bacteria and its naturally occurring byproducts were able to break the gold down into useable nanoparticles. The researchers were even able to watch the microscopic alchemists work without the use of a microscope: as the D. radiodurans changed the size and thus the amount of light absorbed by the gold, the solution in the test tubes changed from a golden yellow to a deep purple.
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| Figure 5. Change in solution color as D. radiodurans transformed gold in solution to Au nanoparticles. |
The resulting gold nanoparticles have incredible potential for adsorbing (a process of collecting and holding) various molecules, especially radioactive ones. Radioactive iodine is incredibly dangerous to living cells and destroys many of the cells and tissues it comes in contact with. As Mi Hee Choi and the rest of the team of researchers learned, it turns out that gold nanoparticles can carry more than cancer drugs: they are also capable of adsorbing isotopes such as radioactive iodine.
This amazing finding seems like it could provide a simple solution to the danger presented by nuclear waste products. However, when gold nanoparticles are placed in an environment that is too salty, the additional molecules cause the nanoparticles to release the hazardous iodine back into the environment. The researchers behind this study noticed the unique applications of both adsorbent nanoparticles and the hardy D. radiodurans and decided to combine them. As prior research has already shown, D. radiodurans is able to transform large gold molecules into smaller nanoparticles, resulting in bacterial cells full of small amounts of the precious metal. D. radiodurans’ ability to both create and hold nanoparticles and withstand intense amounts of radiation makes it the perfect vehicle for gold nanoparticles. Now protected by the bacterial membrane that remains simultaneously permeable and unharmed by radioactive environments, the gold nanoparticles are free to adsorb radioactive iodine without the risk of releasing it. In this way, the D. radiodurans acts as a kind of filter, allowing radioactive isotopes in but not allowing the gold nanoparticles to be harmed or to escape.
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| Figure 6. The combination of nanoparticle synthesis and adsorption of radioactive iodine. |
Similar to the way the solution changed color when D. radiodurans created gold nanoparticles, a change in color in a solution containing Au-DR and radioactive iodine also signals the successful intake of all of the harmful molecules. Given enough time, Au-DR is able to remove 99% of radioactive iodine from water and urine (which you can see in more detail in Figure 6). The efficacy and relative spontaneity of this process means that cleaning an area following exposure to radioactive materials would not require humans to come in direct contact with the cancer-causing molecules. Instead, D. radiodurans and gold nanoparticles can do the dirty work, leaving only an minimally reactive sludge in their wake.
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| Figure 7. Efficiency of bioremediation of Au DR in (a) different solutions containing radioactive iodine, and (b) given varying amounts of the molecule in solution. As time increases, the gold nanoparticles remove more and more of the radioactive iodine from the solution. Even when diluted to 0.1 and 0.01, the gold nanoparticle solution still effectively removes radioactive iodine. |
The results of this study indicate two potential future directions for D. radiodurans. The first is in the use of D. radiodurans for the bioremediation of radioactive iodine, one of the top priority radioactive wastes. The technique developed by these scientists could be efficient and practical in treating radioactive iodine waste. As the first study of its kind, this has opened up a whole new avenue of research in environmental decontamination.
Second, this study was also the first of its kind to present a bioremediation technique using biogenic, gold, nanomaterial-containing microorganisms. It showed that Au-DR has the potential to be used in the bioremediation of radioactive iodine and other metal radioisotopes because of the resistant characteristics of D. radiodurans. This provides additional avenues of research for the application of D. radiodurans in nanotechnology, specifically in the context of bioremediation. As has been seen by this study, D. radiodurans has previously been used in the synthesis of gold (and silver) nanoparticles for biomedical purposes. However, these uses have frequently run counter to goals of bioremediation given the production of contaminants during the process of nanoparticle generation. As such, this presents potential openings for the development of more sustainable nanoparticles and for the conversion of formerly contaminant nanoparticles into decontaminants.
About the Authors:
Sam Neally '19 is an Environmental Studies and Spanish double major with a certificate in Culture, Health, and Science. They’ll be graduating in May and are hoping to use their environmental and bio backgrounds to tackle the ways by which people’s environments influence their health - especially in their home state of Texas! As an ES major, Sam enjoys gallivanting through nature in their free time by partaking in activities like terrorizing Jorge and being terrorized by Jorge.
Mac Chambers '19 is a biology and anthropology double major. During her time at Mount Holyoke, she has asked a lot of questions and has become an expert at skimming abstracts in search for the perfect source of primary lit. Mac hopes to address reproductive healthcare disparities by becoming a certified nurse midwife and earning a degree in community public health. In her free time, Mac enjoys combating antibiotic resistance by licking the floor. Just kidding, she just uses non-antibacterial hand soap and always takes the full course of any prescribed antibiotics.
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| Jorge |
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