Unravelling radionuclide resistance mechanisms in the bacterium Cupriavidus metallidurans

Rogiers Tom

Promoter

Boon Nico, (UGent), nico.boon@ugent.be

SCK•CEN Mentor

Mijnendonckx Kristel
kristel.mijnendonckx@sckcen.be
+32 14 33 21 06

SCK•CEN Co-mentor

Van Houdt Rob
rob.van.houdt@sckcen.be
+32 14 33 27 28

Expert group

Microbiology

PhD started

2017-10-01

Short project description

More than half a century of nuclear activity has globally spread radionuclides in our environment, i.e. soil, water and food. Furthermore, the exposure to radionuclides is potentially increased by human activities via controlled and accidental releases by nuclear power facilities and medical activities. In addition, human activities can lead to an increased exposure to naturally occurring radioactive materials (NORM). Examples of such activities are mining and processing of ores, phosphate industries, production of natural gas or oil.

Microorganisms are often found in radionuclide-contaminated sites where they can influence radionuclide mobility, toxicity and distribution [1, 2]. Key processes are reduction, uptake and accumulation by cells, biosorption and complexation with proteins, polysaccharides and microbial biomolecules, and biomineralization with phosphates and carbonates [3-6]. In turn, long-lived radionuclides can exert a permanent pressure on the prevailing microbial population. Consequently, fundamental understanding of the interaction between microorganisms and radionuclides is essential to correctly assess the microbial impact on the long-term behaviour of radionuclides in contaminated environments. Although the interaction of microorganisms with uranium is extensively studied, there is far less information about the cellular response of microorganisms to uranium exposure. Furthermore, there are only a limited number of studies investigating radionuclides other than uranium.

Cupriavidus metallidurans strains, which are mostly isolated from industrial sites linked to mining, metallurgic and chemical industries, are known for their resistance to a wide plethora of heavy metals. Moreover, type strain C. metallidurans CH34 is used as model organism to study metal resistance. The interaction of C. metallidurans with uranium (U238) and americium (Am241) has been studied [7, 8], however, neither the genes or proteins involved nor the precise mechanism is known. Recently, we showed a strain-specific response and adaptation of C. metallidurans to uranium and evolved a strain in the laboratory to resist increased uranium concentrations.

Objective

The main goal of this PhD project is to pinpoint the gene and gene products that confer uranium resistance in C. metallidurans and, subsequently, to identify the underlying molecular mechanism. In order to determine these key genes, a previously acquired laboratory-evolved strain will be subjected to global genomic and transcriptomic analysis and candidate genes will be selected for further experimental validation via the construction of deletion, overexpression and complementation mutants. Functional analysis of the confirmed genes and their gene products, in combination with microscopy and spectrometry techniques, will allow us to determine the molecular mechanism underlying uranium resistance. In a next phase, the specificity of this response and adaptation to uranium will be evaluated by exposure of C. metallidurans to toxic concentrations of other long-lived natural radionuclides, such as thorium, and artificial radionuclides, such as U233 and americium. In contrast to studies performed until now regarding radionuclide resistance of microorganisms, this PhD will focus on the molecular mechanism driving the resistance and will also study elements other than U238, thereby, functionally identifying new genes and proteins, and pinpointing any radionuclide-specific adaptation process. Understanding the precise cellular response is necessary to investigate their potential in bioremediation purposes.

References
1. Simonoff, M., et al., Microorganisms and migration of radionuclides in environment. Comptes Rendus Chimie, 2007. 10(10-11): p. 1092-1107.
2. Choudhary, S., et al., Uranium and other heavy metal resistance and accumulation in bacteria isolated from uranium mine wastes. Journal of Environmental Science and Health Part a-Toxic/Hazardous Substances & Environmental Engineering, 2012. 474): p. 622-637.
3. Ding, C.C., et al., Biosorption behavior and mechanism of thorium on Streptomyces sporoverrucosus dwc-3. Journal of Radioanalytical and Nuclear Chemistry, 2014. 301(1): p. 237-245.
4. Payne, R.B., et al., Interaction between uranium and the cytochrome c(3) of Desulfovibrio desulfuricans strain G20. Archives of Microbiology, 2004. 181(6): p. 398-406.
5. Merroun, M.L. and S. Selenska-Pobell, Bacterial interactions with uranium: An environmental perspective. Journal of Contaminant Hydrology, 2008. 102(3-4): p. 285-295.
6. Macaskie, L.E., B.C. Jeong, and M.R. Tolley, Enzymically accelerated biomineralization of heavy metals: application to the removal of americium and plutonium from aqueous flows. FEMS Microbiol Rev, 1994. 14(4): p. 351-67.
7. Llorens, I., et al., Uranium Interaction with Two Multi-Resistant Environmental Bacteria: Cupriavidus metallidurans CH34 and Rhodopseudomonas palustris.Plos One, 2012. 7(12).
8. De Borba, T.R., et al. Application of Bacteria to Remove Americium from Radioactive Liquid Waste. In WM2011 Conference. 2011. Phoenix, AZ.