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APPLICATIONS

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Thermophilic radiation-resistant microorganisms have a wide range of applications ranging from bioremediation, astrobiology, and radiation therapies to basic and applied biology studies. Figure 10.4 presents an overview of different applications of radiation-resistant thermophiles.

US Department of Energy (DOE) originated the idea of engineered bioremediation where engineered strains of D. radiodurans were used for cleaning up radioactive environmental waste left from nuclear weapons (Macilwain, 1996). The engineering of D. radiodurans was significant to treat radionuclide wastes but the temperature restricted bioremediation process to 39°C as the contaminated waste was thermally insulated and the engineering of a radiation-resistant thennophile was essential to decay radionuclides like 137Cs and 90Sr (Lange et al., 1998; Brim et ah, 2000, 2003; Daly, 2000). Brim et ah (2003) reported efficient expression of gene systems developed for D. radiodurans in thennophile D. geothermalis thus, gene technology developed for D. radiodurans was readily transferrable to D. geothermalis. This could be applied in cleaning up radioactive environments, which usually works at high temperatures. The wild and engineered D. geothermalis has the ability to reduce number of metals, viz., U(VI), Cr(YI), Hg(II), Tc(VH), Fe(III) and Mn(III, IV) so US DOE predicted that it can be used for designing of efficient bioremediation systems which include cleaning up of radioactive wastes sites (Brim et ah, 2000, 2003, 2006; Daly, 2000; Fredrickson et ah, 2000). It is proposed that!), geothermalis strain T27 can be used for cleanup of xenobiotic compounds from contaminated environments as this strain has the ability to survive at elevated concentrations of diethylpthalate, ethyl acetate and toluene (Kongpol et ah, 2008). Other radiation-resistant thermophiles that have made their way to environmental cleaning technology are thermophilic bacteria like Pyrobaculum islandicum, Tepidibacter thalassicus, and Thermoterrabacterium ferrireducens and archaea like Thermococcus pacificus and Thermoproteus uzoniensis can reduce "Тс (VII) to insoluble Тс (IV) precipitates and thus immobilizing Tc, a p-emitting fission product of 235U which can be hazardous as it gets accumulated in food chain (Chernyh et ah, 2007; Sar et ah, 2013). Thus, thermophilic radiation-resistant microbes, either in their wild type or recombinant form, are the best candidates for environmental cleanup of radioactive wastes contaminated sites (Chernyh et ah, 2007; Sar et ah, 2013).

The radiation-resistant thermophiles are finding an important place in astrobiological studies. The recovery of Deinococcus plioenicis from phoenix spacecrafts indicated that microbes are ubiquitous (Stepanov et ah, 2014). Life in outer space encounters multiple stresses, viz., high IR and temperature variation from hot to cold or vice versa. Hie radiation- resistant thermophiles, being a robust organism can survive high temperature, IR, desiccation, and even reported to be brought back to life even after 250 million years indicated that life could be extraterrestrial in origin (Vreeland et al., 2000; Rampelotto, 2010). Thus, exploration of life that thrives at high temperatures may lead to exciting discoveries and would also pave the path for astrobiological studies to relate high- temperature radiation-resistant survivors as the candidates that originated life on Earth.

Applications of radiation-resistant thermophiles. Source

FIGURE 10.4 Applications of radiation-resistant thermophiles. Source: Modified from Ranawat and Rawat, 2017b.

Compounds like amino acids, betain, sugar, and heteroside derivatives synthesized by extremophilic microorganisms help them to survive in high radiations (Letzen and Schwaz, 2006; Graf et al., 2008). These compounds are “extremolytes” synthesized by radiation-resistant microbes and can be used for pharmaceutical as well as biotechnological industries (Singh and Gabani, 2011). It is also hypothesized that extemolytes from ultraviolet radiation (UY-R)-resistant microorganisms can be employed for the development of anticancer drags to prevent skin damage from UY-R as these molecules are inert in nature and also resist effects of radiations. Thus, metabolites from UV-R-resistant microorganisms could be a possible source of efficient human therapeutics which include anticancer drags, anti-oxidants, and cell cycle blocking agents (Singh and Gabani, 2011). Pavlopoulou et al. (2016) reported that proteins of two families play a crucial role in enhancing radiation resistance. These include DNA histone-like DNA binding protein HU, which can condense DNA and form nucleoid-like structure and the other one is group of DNA repair proteins. The new insights into the radiation resistance biology can be provided by coupling analytical structural analysis with next-generation sequencing so that new parameters which confer radiation resistance in thermophilic microorganisms could be unveiled. These studies will help to develop ways and strategies to overcome tolerance to IR and improve radiation therapies.

Radiation resistant thennophiles like D. geothermalis, D. radioduraus and Sidfolobus solfatricus are studied for transfer and expression of genes in mesophiles like E. coli (Brim et al., 2003). D. radioduraus was engineered for bioremediation by cloning the mer operon of E. coli on plasmid. This plasmid cloned and expressed in D. radiodurmis was successfully functional in D. geothermalis, which was able to grow at 50°C and could reduce mercury at high temperatures (Brim et al., 2003). The aspATS gene of thermophilic Sulfolobus solfatricus, which code for enzyme aspartate aminotransferase gene, was expressed in E. coli (Amone et al., 1992). This suggests the complexity of mechanisms governing thennophilicity and radiation resistance and also provides evidence which suggests the exchange of genes between mesophilic and thermophilic microorganisms. Moreover, the study of radiation-resistant thennophiles can be effectively employed in the study of molecular switches like operons and regulon. There are platfonns like “Radio PI” - a database of radiation resistance prokaryotes that provide a vast knowledge of radiation-resistant microorganisms, including thennophiles (Ben-Hamda et al., 2015). It presents an overview of predicted genes that are present during oxidative stress, DNA repair mechanisms, and potential uses of radiation-tolerant prokaryotes in biotechnology. These databases are important for the studies dedicated to evolutionary biology, therapeutics, and biotechnological applications of radiation-resistant thennophiles. This can be achieved by linking databases with the Kyoto Encyclopedia of Genes and Genomes (KEGG) and COGs (Cluster of Orthologs Genes), which will further analyze prokaryotes and categorize functional units from a new perspective (Ben-Hamda et al., 2015).

CONCLUSION

Thermophiles are the most interesting and most explored extremophiles. These microorganisms have drawn enormous attention from the scientific community due to their potential to produce extremozyes. However, the applications of thermophiles are not only restricted to the production of thermostable enzymes, but these microorganisms also have applications in bioremediation of heavy metals and radionuclides, textile effluents, antibiotic production, and so on. The discoveiy of thermophilic radiation- resistant members of genera Deinococcus and Rubrobacter has attracted the attention of the scientific world to understand the mechanisms which assist these microorganisms to deal with multiple stresses simultaneously. Radiation-resistant thermophiles, viz., D. geothermalis, Rubrobacter spp., are used as model microorganisms in various research laboratories to study the effects of cytotoxic compounds formed due to an exposure to radiations and cause damage to DNA and proteins. The understanding of mechanisms which govern radiation resistance in thermophiles has increased the application of these microorganisms and now they can be employed for bioremediation of environments which are thermally insulated and contaminated with radionuclide wastes. In the future, with the help of recombinant DNA technology, mesophilic microorganisms can be engineered by the transfer of radiation-resistant machinery of thermophiles, and thus, mesophiles can also be employed for cleanup of sites with radioactive wastes. The radiation resistance and high temperature in these microbes would provide an astrobiological link of thermophiles and pave the way for their possible use in radiation therapies to treat skin cancer. System biology-based approaches like genomics, proteomics, metagenomics, and metatranscriptomics are necessary to further develop the knowledge on the stress biology of thermophilic radiation-resistant microorganisms. This will open up horizons for exploring thermophiles and provide insights into radiation biology and radiation resistance in thermophiles.

KEYWORDS

  • Deinococcus geothermalis
  • Deinococcus radiodurans
  • • desiccation resistance
  • • ionizing radiations
  • • metatranscriptomics
  • • proteomics
  • • reactive oxygen species
  • Rubrobacter xylanophilus
  • Sulfolobus solfatricus
  • Taq polymerase
  • • thermophiles
  • Thermus thermophilus
 
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