Nuclear Microbiology

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UVA, UVB, and UVC radiation effects on a bacterial cell

Some microorganisms exhibit genetic and molecular adaptations which allow them to withstand extreme exposure to radiation. These adaptations can be used in a variety of applications such as bioremediation and medicinal therapeutics.

Organisms are exposed to multiple types of radiation on a daily basis. The most common form of radiation is Ultraviolet. UV radiation is emitted from the sun and can be broken down into three types: UVA (320-400nm), UVB (280-320nm), and UVC (<280nm). These wavelengths produce enough energy to damage the genetic integrity of many organisms. Exposure to UVB and UVC radiation results in the formation of pyrimidine dimers and reactive oxygen species[1]. Organisms found at high altitudes experience the highest levels of UV exposure, and are therefore some of the most radiation-resistant organisms. Radiation-resistant microbes are also commonly found in radioactively contaminated locations.

Halobacterium sp. NRC-1

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Halobacterium sp. NRC-1 is able to withstand solar radiation by inducing the phr gene which reverses the products of UV radiation. And it also induces the radA gene, which is related to recA, a gene which is directly involved in homologous recombination[2].

In response to exposure to high energy gamma radiation, which can result in double-stranded DNA breaks, Halobacterium produces membrane pigments in order to protect the genome against damage, and also induces the rfa3 gene, which encodes for a protein involved in DNA repair[3].

Deinococcus radiodurans

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Deinococcus radiodurans is known to be the most radiation resistant organism discovered in modern microbiology. It displays extreme resistance to both ionizing and UV radiation. An understanding of the mechanisms of such resistance would allow for its use in biotechnological applications such as bioremediation. Molecular alterations to historically heat resistant systems may have resulted in its ability to survive exposure to extreme irradiation[4]. Findings from comparative genetic analysis between Deinococcus radiodurans and Thermus Thermophilus showed that they are closely related on a phylogenetic level. Comparisons on the protein level indicated that they are both descendants of thermophilic ancestors. This supports the notion that the radiation resistant system in Deinococcus radiodurans could be based on a previously heat resistant system. The specific mechanisms in which Deinococcus radiodurans is able to withstand irradiation are through the protection against reactive oxygen species (ROS) damage post-exposure, as well as utilization of adequate DNA repair systems to ensure integrity of the genome and avoid genetic mutations[5][6].

 
Incorporation of merA gene from E. coli in D. radiodurans allows for radioactive Mercury, Hg (II) reduction

D. radiodurans can be used in various bioremediation applications involving the breakdown of toxic elements in nuclear waste sites. An example where this bacterium can be used for bioremediation application is in nuclear weapons production bases containing high contents of Mercury, Hg (II). One mechanism for this application involves the insertion of a known mercury resistance gene (merA) from Esherichia coli into several strains of D. radiodurans. The engineered strains act to reduce ionic Mercury, Hg (II) found in these waste sites[7]. Prospectively, D. radiodurans can potentially be used for other bioremediation applications if engineered to breakdown specific toxic compounds using the corresponding genetic manipulation.

Geobacter sulfurreducens

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Uranium is a naturally occurring radioactive element. This element is commonly found from leakage of nuclear plants as well as granite erosion[8]. Once free uranium is found in groundwater it becomes oxidized immediately to uranium (VI). When this oxidized element is prevalent in groundwater it becomes extremely hard to remove due to forming complexes with other uranium (VI) resulting in uranyl cations. The only way to remove the Uranium (VI) is to reduce it to uranium (IV) in order for it to become less soluble in groundwater and is able to precipitate out of solution.

Geobacter sulfurreducens is a bacterium that has the ability to generate biofilms as well as conductive pili used to reduce metals. This bacterium has a positive correlation in biofilm development and uranium (VI) reduction. G. sulfurreducen secrets extra polymeric substances that form the biofilm. Once this biofilm has developed a larger enough surface area it is able to secrete cytochrome’s into the biofilm. Cytochrome OmcZ is involved in carrying electron across the biofilm. Once a metal such as uranium comes in contact with these biofilms uranium (VI) is reduced to soluble uranium (IV).

The second component of this bacterium to reduce uranium (VI) by using of the pili found on the outer membrane. This pili is encoded by the pilA gene and is tied to bioremediation of uranium. When this pilus is not made uranium reduction significantly decreases. The cytochrome OmcS c-cytochrome is commonly found in the pili. This cytochrome is involved in the transfer electrons from the cell and Fe(III) oxidases. This pilus conducts electron from an organic carbon source such as acetate using it as an electron donor in order to conduct the electron onto the pili which is then used to reduce metals such as uranium when coming in contact[9].

Bacillus cereus

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High altitude lakes Laguna Negra and Laguna Verde in Argentina consist of high UV-B radiation with wavelength of 280-320nm. Cyclobutane pyrmidine dimers occur when DNA is in contact with radiation at these levels. UV radiation leads to reactive oxygen species causing further DNA damage. Bacillus cereus was identified to able survive these radiation levels based on their pigmentation and the bacterium ability to produce astaxan. Astaxan is a carotene pigment which absorbs wavelength of 470nm which is then used to synthesize ROS scavenging proteins. It was noted that these bacteria only produce this pigment when they are exposed to radiation and are in the presence of LM media found in these lakes[10].

Ascophyllumnodosum

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Phlorotannin, a compound produced by Ascophyllumnodosum in response to UVR exposure, may be used to target free radicals as a therapeutic intervention against oxidative stress. Oxidative stress can also be targeted by utilizing Usurijene, a compound produced by Synechocystis sp. following UVR exposure. Usurijene acts against H2O2, which is a potent reactive oxygen compound known to cause oxidative stress[1][11]. Other compounds such as Palythine and mycosporine-taurine, which are also produced by some microorganisms in response to ultraviolet radiation exposure, can be used as sunscreens for their ability to withstand high levels of radiation[1].

Therapeutic implications

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Microbial adaptations of marine organisms to extreme conditions can be used for potential anticancer therapies. The byproducts of extremophiles, also known as extremolytes, following UVR exposure can be utilized for other medical applications as well[1].

Bacteria surviving in the oceans are also susceptible to UV radiation. One bacteria that is able to survive in conditions of UV radiation exposure is Prochloroccous. UV radiation exposure leads to cyclobutane pyrimidine dimers and oxidative stress. This bacterium is able to survive these conditions by two genes phrB and mutT and phrB. PhrB is a photolyase which is used to break down pyrimidine dimers formed from UV radiation. MutT has been identified as a nudix hydrolase. This hydrolase becomes active during oxidative stress, which activates 8-oxo-dGTP to removes reactive oxygen species that attaches onto guanine in DNA[12].

References

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  1. ^ a b c d Singh, O. V.; Gabani, P. (April 2011). "Extremophiles: radiation resistance microbial reserves and therapeutic implications". Journal of Applied Microbiology. 110 (4): 851–861. doi:10.1111/j.1365-2672.2011.04971.x. ISSN 1365-2672. PMID 21332616.
  2. ^ Crowley, David J.; Boubriak, Ivan; Berquist, Brian R.; Clark, Monika; Richard, Emily; Sullivan, Lynn; DasSarma, Shiladitya; McCready, Shirley (2006-09-13). "The uvrA, uvrB and uvrC genes are required for repair of ultraviolet light induced DNA photoproducts in Halobacterium sp. NRC-1". Saline Systems. 2: 11. doi:10.1186/1746-1448-2-11. ISSN 1746-1448. PMC 1590041. PMID 16970815.{{cite journal}}: CS1 maint: PMC format (link) CS1 maint: unflagged free DOI (link)
  3. ^ "Extreme halophiles are models for astrobiology". Microbe-American Society for Microbiology. 1(3), 120.
  4. ^ White, O.; Eisen, J. A.; Heidelberg, J. F.; Hickey, E. K.; Peterson, J. D.; Dodson, R. J.; Haft, D. H.; Gwinn, M. L.; Nelson, W. C. (1999-11-19). "Genome sequence of the radioresistant bacterium Deinococcus radiodurans R1". Science (New York, N.Y.). 286 (5444): 1571–1577. ISSN 0036-8075. PMC 4147723. PMID 10567266.{{cite journal}}: CS1 maint: PMC format (link)
  5. ^ Omelchenko, Marina V; Wolf, Yuri I; Gaidamakova, Elena K; Matrosova, Vera Y; Vasilenko, Alexander; Zhai, Min; Daly, Michael J; Koonin, Eugene V; Makarova, Kira S (2005-10-20). "Comparative genomics of Thermus thermophilus and Deinococcus radiodurans: divergent routes of adaptation to thermophily and radiation resistance". BMC Evolutionary Biology. 5: 57. doi:10.1186/1471-2148-5-57. ISSN 1471-2148. PMC 1274311. PMID 16242020.{{cite journal}}: CS1 maint: PMC format (link) CS1 maint: unflagged free DOI (link)
  6. ^ Jones, Daniel L.; Baxter, Bonnie K. (2017). "DNA Repair and Photoprotection: Mechanisms of Overcoming Environmental Ultraviolet Radiation Exposure in Halophilic Archaea". Frontiers in Microbiology. 8: 1882. doi:10.3389/fmicb.2017.01882. ISSN 1664-302X. PMC 5626843. PMID 29033920.{{cite journal}}: CS1 maint: PMC format (link) CS1 maint: unflagged free DOI (link)
  7. ^ Brim, Hassan; McFarlan, Sara C.; Fredrickson, James K.; Minton, Kenneth W.; Zhai, Min; Wackett, Lawrence P.; Daly, Michael J. (2000-01). "Engineering Deinococcus radiodurans for metal remediation in radioactive mixed waste environments". Nature Biotechnology. 18 (1): 85–90. doi:10.1038/71986. ISSN 1087-0156. {{cite journal}}: Check date values in: |date= (help)
  8. ^ "Environments Well Water Uranium and Your Health - CDC Tracking Network". ephtracking.cdc.gov. Retrieved 2018-04-29.
  9. ^ "The biogeochemistry and bioremediation of uranium and other priority radionuclides". Chemical Geology. 363: 164–184. 2014-01-10. doi:10.1016/j.chemgeo.2013.10.034. ISSN 0009-2541.
  10. ^ Flores, María R.; Ordoñez, Omar F.; Maldonado, Marcos J.; Farías, María E. (December 2009). "Isolation of UV-B resistant bacteria from two high altitude Andean lakes (4,400 m) with saline and non saline conditions". The Journal of General and Applied Microbiology. 55 (6): 447–458. ISSN 0022-1260. PMID 20118609.
  11. ^ Coyle, Christian H.; Kader, Khalid N. (January 2007). "Mechanisms of H2O2-induced oxidative stress in endothelial cells exposed to physiologic shear stress". ASAIO journal (American Society for Artificial Internal Organs: 1992). 53 (1): 17–22. doi:10.1097/01.mat.0000247157.84350.e8. ISSN 1538-943X. PMID 17237644.
  12. ^ Osburne, Marcia S; Holmbeck, Brianne M; Frias-Lopez, Jorge; Steen, Robert; Huang, Katherine; Kelly, Libusha; Coe, Allison; Waraska, Kristin; Gagne, Andrew (2010-7). "UV hyper-resistance in Prochlorococcus MED4 results from a single base pair deletion just upstream of an operon encoding nudix hydrolase and photolyase". Environmental Microbiology. 12 (7): 1978–1988. doi:10.1111/j.1462-2920.2010.02203.x. ISSN 1462-2912. PMC 2955971. PMID 20345942. {{cite journal}}: Check date values in: |date= (help)CS1 maint: PMC format (link)