Research about Deinococcus radiodurans

Deinococcus radiodurans

I. Introduction

Deinococcus radiodurans is an extremophilic bacterium,one of the most radiation-resistant organisms known. It can survive cold, dehydration, vacuum, and acid and is therefore known as a polyexremophile and has been listed as the world's toughest bacterium in The Guinness Book of World Records.

Deinococcus radiodurans was discovered in 1956 by an American researcher when he intended to sterilize beef canned foods by irradiating an intense gamma radiation. He was surprised to note that bacteria did not die below this radiation level. Deinococcus radiodurans is able to withstand radiation levels hundreds of times higher than lethal doses.

The gene of Deinococcus radiodurans has a ring-shaped structure that breaks into thousands of pieces. The researchers noted that the cells appear to die in about an hour and a half. But three hours after the radiation, its DNA was reassembled.

French researcher Miroslav and colleagues found that this restoration was divided into two phases. The first is that the cracked pieces remove all of the broken heads and put them together. The pieces are used as models to synthesize DNA and form a long string. After this stage of gene recombination, the strands join together to recreate chromosomes. Deinococcus radiodurans is Deinococcus radiodurans is a gram-positive, nonsporulating bacterium which usually grows in tetrad form.

II. Bioremediation 

Recombinant D. radiodurans cells for bioremediation of heavy metals from acidic/neutral aqueous wastes. The stability and superior metal bioremediation ability of genetically engineered Deinococcus radiodurans cells, expressing a non-specific acid phosphatase, PhoN in high radiation environment has already been established. The lyophilized recombinant DrPhoN cells retained PhoN activity and uranium precipitation ability. Such cells also displayed an extended shelf life of 6 months during storage at room temperature and showed surface associated precipitation of uranium as well as other metals like cadmium. Lyophilized cells, immobilized in polyacrylamide gels could be used for uranium bioprecipitation in a flow through system resulting in 70% removal from 1mM input uranium solution and a loading of 1 g uranium/g dry weight cells. Compared with a batch process which achieved a loading of 5.7 g uranium/g biomass, the efficiency of the column process was low due to clogging of the column by the precipitate.

a. Application of DrPhoN Cells for Metal Removal

Our earlier studies using the recombinant E. coli (EcPhoN) and D. radiodurans (DrPhoN) cells expressing the phoN gene from the deinococcal PgroESL promoter showed that both the in-gel as well as the cell bound PhoN activities were higher in recombinant cells of E. coli than in Deinococcus. A possible explanation for this may lie in the six layered cell wall which D. radiodurans is known to possess. The precise localization of the PhoN enzyme among these six layers is not known since the periplasm is poorly defined in Deinococcus. But, this may limit access of substrate and its availability and result in lower activities.

b. Surface Bioprecipitation of Metals Circumvents Metabolic Toxicity of Metals

Cell surface association of the bioprecipitated uranium was confirmed in DrPhoN cells by scanning electron microscopy wherein the uranyl phosphate precipitate appeared as small needle like structures covering the entire cell surface. It has been hypothesized that cell associated metal precipitation is initiated at nucleation sites present on the cell surface. In Citrobacter, the high content of phosphates in extracellular polysaccharide acts as complexation sites for the incoming metal ion and the initial nucleation site is consolidated by continuous addition of phosphate ligand generated by the enzymatic process. The fact that a variety of organisms tested so far, including E. coli, D. radiodurans and Sphingomonas could bring about cell bound metal precipitation indicates that the cell surface structures required for metal precipitation are not very specific but are of a more general character, across different bacteria and sufficient for efficient metal precipitation and loading.

c. Uranium Precipitation: Batch vs. Flow-Through Process

With the objective of simplifying environmental application of recombinant PhoN expressing bacteria for metal bioremediation, cells were subjected to lyophilization. Lyophilized EcPhoN and DrPhoN cells retained phosphatase activity as well as uranium precipitation ability for up to six months of storage at room temperature. Further, such lyophilized cells could be immobilized in polyacrylamide gels and packed into columns to construct a flow-through system for uranium precipitation. When gravity based flow-through column was used for uranium precipitation, a loading of 0.73 g uranium/g dry weight of biomass was achieved. An improvement in operation of the column was attempted by using a bigger column (2.5 cm I.D × 50 cm H, 90 ml void volume) and passing the assay solution upwards using a peristaltic pump. The results indicated that lyophilized cells (immobilized in acrylamide) remained stable throughout the operation of the column and over a long period of time. Nearly 70% removal of the input uranium concentration could be achieved when the column was operated at a flow rate of 38 ml/h.

III. Conclusion

The superior uranium precipitation ability of DrPhoN cells in high radiation environments had already been established. The ability to precipitate other toxic metals like cadmium, amenability for use in batch and continuous process and improved shelf life and ease of application achieved through lyophilization have been recent value additions to this strain. A number of further improvements are desirable to bring PhoN based metal precipitation technology to its full potential. These include (a) alternative, cheap substrate for phosphatase in place of β-glycerophosphate in order to make the process economically viable, (b) engineering the phosphatase to localize closer to the cell surface especially in an organism like D. radiodurans where multi-layered cell walls may seriously limit the substrate availability and metal access to the enzyme and (c) recombinant Deinococcus cells also need to be tested for a wide array of metals for even non-nuclear applications, such as in nickel cadmium battery waste clean-up. These possibilities are currently being investigated.

Works cited

Bugs, Bioeng. "Recombinant D. radiodurans cells for bioremediation of heavy metals from acidic/neutral aqueous wastes." 2012 Jan 1; Epu, https://www.ncbi.nlm.nih.gov/pubmed/22179144#. Assessed on 11/16/2019.