A Microbial Biorealm page on the genus Ralstonia metallidurans
Higher order taxa
Class: Beta Proteobacteria
Species: R. metallidurans
Description and significance
Ralstonia metallidurans is a gram-negative microbe that does not form spores. It is unique in that it can fluorish in millimolar concentrations of heavy metals, such as gold, that are normally toxic to bacteria. According to discoverer Frank Reith, Ralstonia metallidurans "is able to survive in concentrations of gold that would kill most other micro-organisms" (1). Not only can these microbes withstand toxic concentrations of metals, they are found to be able to precipitate gold. Although it is unknown exactly how this process works, it is "possible that the microbe screens out gold as part of an effort to detoxify its immediate environment" (1).
Ralstonia metallidurans and other metal resistant microbes are often found in sediments with a high content of heavy metals from diverse geographical locations. The reference strain, CH34, was first discovered in 1976, from the sludge of a zinc decantation tank in Belgium that was polluted with high concentrations of several heavy metals. Frank Reith (and colleagues) at Australian National University further isolated and grew the bacteria in the lab.
The significance of this microbe lays in that its genome contains plasmids that confer resistance to heavy metals, such as Zinc, Mercury, Cadmium, Cobalt, etc. In 1992, Kostman et al. developed a PCR-ribotyping method for B. cepacia which could detect significant polymorphisms in the intergenic 16S-23S spacer of rRNA genes (7). In 1997, Shreve et al. used PCR-RFLP analysis for the 16S and 23S regions of the rRNA genes in an epidemiological study of B. cepacia infection (21). Segonds et al. used PCR-RFLP analysis of the 16S rRNA gene to differentiate Burkholderia species (20).
The genome of R. metallidurans CH34 has been extensively studied. It contains two large plasmids that encode for heavy metal resistance. pMOL28 (163 kb) furnishes tolerance to Nickel, Mercury, and Cobalt; pMOL30 (238 kb) confers resistance to Zinc, Cadmium, Mercury, and Cobalt (3). These plasmids are self-transmissible in homologous matings, but at low frequencies (3). The entire genome is approximately 6900 kb in size. In addition to the plasmids pMOL28 and pMOL30, it also contains two circular chromosomes (8). Furthermore, the genome encodes "8 P-type ATPase involved in metal efflux specialized in lead, cadmium, thallium and/or copper efflux, and several other mechanism involved in metal processing" (1). Together, these genetic elements contribute to the microbe's unique ability to survive in harsh environments. The amplicons of R. paucula isolates were digested with AciI and AlwI. All of the strains shared a single pattern with each of the two endonucleases. The AciI pattern was composed of eight fragments (340, ~200, 122, 119, 92, 75, 49, and 44 bp), and the AlwI pattern was composed of four fragments (418, 218, 130, and 75 bp)
Cell structure and metabolism
R. metallidurans is a gram-negative bacillus (rod shaped). Therefore, it has the structural features of gram-negative bacteria, such as cell walls containing peptidoglycan, an outer membrane containing LPS and porins, and a periplasmic space (2).
R. metallidurans is able to utilize a variety of substrates as its carbon source. It can grow autotrophically using molecular hydrogen as an energy source and carbon dioxide as its carbon source. Furthermore, in the presence of nitrate, it can grow anaerobically (6). When grown lithoautotrophically on molecular hydrogen, it forms cytoplasmic NAD-reducing and membrane bound hydrogenase. It does not grow on fructose (3) and its optimal growth temperature is 30 C (4).
Due to its ability to withstand toxic metals, R. metallidurans has been studied to utilize this characteristic in areas of bioremediation (10). Furthermore, new findings of a lead binding protein isolated from R. metallidurans is key in preventing lead poisoning (11).
R. metallidurans has not been found to be pathogenic to humans. However, a near relative, R. solancearum, that shares some of the metal resistant genes, has been found to cause disease in plants (10). R. paucula has been isolated from pool water, groundwater, and bottled mineral water (2, 9, 17) and from clinical specimens. Despite its low pathogenicity, it is now recognized as an opportunistic pathogen which can generate serious infections such as septicemia, peritonitis, abscess, and tenosynovitis, particularly in immunocompromised patients
Application to Biotechnology
R. metallidurans has been found to produce enzymes that may be used in constructing a fuel cell (12). These enzymes are able to oxidize hydrogen, which can ultimately result in the production of electricity. Scientists are in the process of researching this phenomenom (12).
Currently, R. metallidurans are being studied for their potential in fuel cell production (12). Also, as mentioned above, this microbe has the unique ability to precipitate gold. Current research is trying to better understand this interesting characteristic(1). Furthermore, R. metallidurans has been studied for its use in the prevention of lead contamination by the fluorescent signaling of its lead binding protein (11). Recently, several authors have reported the use of PCR-ribotyping (3, 5, 7, 8, 12, 22, 23) and PCR-restriction fragment length polymorphism (PCR-RFLP) analysis (20, 21) for strain differentiation within various bacterial species. Thus, an evaluation for the ability of these two techniques to distinguish between R. paucula strains and between strains belonging to other Ralstonia species (R. eutropha, R. pickettii, R. gilardii, and R. solanacearum) is going on. As the ribosomal DNA (rDNA) intergenic spacer can contain genes coding for tRNA (15, 24), we also explored this region in R. paucula and compared it to those of other Ralstonia species.
1.Llyod, Robin. "Eureka! Bacteria Have the Midas Touch" 18 July 2006. www.livescience.com/strangenews/060718_gold_bacteria
2.Larsen, Rachel and Kit Pogliano. "Outside structures 1: cell wall and membranes." BIMM 120: Introductory Microbiology. University of California, San Diego. Spring 2007.
3. M Mergeay. "Alcaligenes eutrophus CH34 is a facultative chemolithotroph with plasmid-bound resistance to heavy metals." J Bacteriol. April 1985 http://jb.asm.org/cgi/content/abstract/162/1/328
4. "Cupriavidas metallidurans CH34" JGI Microbes http://genome.jgi-psf.org/draft_microbes/ralme/ralme.home.html
10. M Mergeav. "Ralstonia metallidurans, a bacterium specifically adapted to toxic metals: towards a catalogue of metal-responsive genes." FEMS Microbial Rev. June 2003 http://ncbi.nlm.nih.gov/sites/entrez?
12. American Chemical Society."New 'biofuel cell' produces electricity from hydrogen in plain air."26 March 2007. http://www.physorg.com/news94144517.html
Edited by Shu-Mei (April) Yu student ofRachel Larsen and Kit Pogliano.