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Nuclear Microbiology

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. 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

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.

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.

Deinococcus radiodurans

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. 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.

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. 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

Uranium is a naturally occurring radioactive element. This element is commonly found from leakage of nuclear plants as well as granite erosion. 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.

Bacillus cereus

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.

Ascophyllumnodosum

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. 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.

Therapeutic implications

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.

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.