Ralstonia metallidurans

A Microbial Biorealm page on the genus Ralstonia metallidurans

Bacterial induced formation of Cd crystals by Ralstonia metallidurans on Zirfon M5 membranes in the continuous tubular membrane reactor.. [1]

Classification

Higher order taxa

Kingdom: Bacteria

Phylum: Proteobacteria

Class: Beta Proteobacteria

Order:Burkholderiales

Family: Ralstoniaceae

Genus: Ralstonia

Species: R. metallidurans

Strain: CH3

Species

NCBI: Taxonomy

Ralstonia basilensis
Ralstonia campinensis
Ralstonia eutropha
Ralstonia gilardii
Ralstonia insidiosa
Ralstonia mannitolilytica
Ralstonia paucula
Ralstonia pickettii
Ralstonia respiraculi
Ralstonia solanacearum
Ralstonia syzygii
Ralstonia taiwanensis

Description and significance

Alcaligenes eutrophus CH34 was just recently renamed Ralstonia metallidurans. This species was first isolated in 1976 from the sludge of a zinc decantation tank in Belgium that was polluted with high concentrations of several heavy metals, it and other metal-resistant members of the genus Ralstonia are frequently found in sediments and soils with a high content of heavy metals from diverse ecological niches(2).

Ralstonia metallidurans is gram-negative, non-spore forming bacterium which thrives in the presence of millimolar concentrations of heavy-metals such as Zn, Cd, Co,Pb, Hg, Ni and Cr. Metal resistance functions are mainly encoded on two plasmids replicated in the bacterium, pMOL28 and pMOL30, which produce metal exporters that pump metal ions out of the cell. This process aids in protecting intracellular macromolecules from the toxic effects and damages of high concentrations of metal (2).

This extremophile is better able to withstand high concentrations of heavy metals than any other well-studied organism. This fact renders it a potential agent of bioremediation as well as an ideal model organism for understanding metal resistance phenotypes. Due to its survival in extreme metal concentrations and genomic sequencing it is thought to be one of the first life forms on earth, existing when there was very little oxygen in the atmosphere some 2.5 billion years ago.

Genome structure

Its genome is approximately 6800 kb in size. The type strain CH34 carries two large circular plasmids(pMOL28=180 kbp and pMOL30=240 kbp)and two circular chromosomes bearing a variety of genes for metal resistance and carrying gene clusters that encode cation-efflux machinery spanning both bacterial membranes. Strain CH34 contains two plasmids, pMOL28 (163 kilobases) specifying nickel, mercury, and cobalt resistance and pMOL30 (238 kilobases) specifying zinc, cadmium, mercury, and cobalt resistance.

. Circular genome of plasmid 1 for Ralsontia metallidurans found using [tools.neb.com]

The plasmids are self-transmissible in homologous matings, but at low frequencies. The transfer frequency was strongly increased with IncP1 plasmids RP4 and pUZ8 as helper plasmids. Plasmid-free cells of strain CH34 are still able to grow lithoautotrophically and to form both hydrogenases, indicating that the hydrogenase genes are located on the chromosome, in contrast to the Hox structural genes of strain H16, which are located on the megaplasmid pHG1 (450 kilobases). This genome also contains 8 P-type ATPase involved in metal efflux specialized in lead, cadmium, thallium and/or copper efflux, and several others mechanisms involved in metal processing.

Cell structure and metabolism

Ralstonia metallidurans uses a variety of substrates as its carbon source. It can grow chemo-lithotropically using molecular hydrogen as the energy source and carbon dioxide as a carbon source to form a cytoplasmic NAD-reducing and a membrane-bound hydrogenase. It contains most metabolic attributes; however, it does not grow on fructose(5).R. metallidurans can reduce selenite to elemental red selenium, metabolize glutamatee, ascorbate, aldarate and nitrogen. When nitrate is present Ralsonia metallidurans can grow anaerobically. Its optimal growth temperature is 30 çC.

Ralstonia metallidurans is rod-shaped, motile aerobe. It is a gram-negative bacteria and pocesses such traits as the cell walls contain peptidoglycan; Cells are surrounded by an outer membrane containing lipopolysaccharide (5); Porins exist in the outer membrane, which act like pores for particular molecules (5); There is a space between the layers of peptidoglycan and the secondary cell membrane called the periplasmic space (5); The S-layer is directly attached to the outer membrane, rather than the peptidoglycan; No teichoic acids or lipoteichoic acids are present.

Cell structure of a gram-negative bacterium such Ralstonia metalidurans .. [www.scq.ubc.ca/.../08/cationicpeptides(1).gif]

Ecology

Due to its extremophilic ability to withstand high concentrations of heavy metals it renders it as a potential agent of bioremediation of soil and water contaminated with these toxic heavy metals or chlorinated organic compounds. It also points towards being an ideal model organism for understanding metal resistance phenotypes.

Lead contamination in our food and water is a serious threat to human health and the environment. Typically, lead levels are measured by using atomic absorption spectroscopy. The recenent finding of a fluorescent Lead II Probe from Lead II-regulatory protein isolated from Ralstonia metallidurans could provide rapid, on-site evaluation of the lead content of a sample.

Pathology

Ralstonia metallidurans is not a pathogen and does not inhabit any host organisms. However, many other species of Ralsontonia can be pathogenic, some to plants (Ralstonia solanacearum)
and even others that have been seen in cystic fibrosis patients(Ralstonia pickettii). Many species of Gram-negative bacteria like R. metallidurans are pathogenic to other organisms. This pathogenic capability is usually associated with the structural components of gram-negative cell walls, in particular the lipopolysaccharide (also known as LPS or endotoxin) layer. The LPS is the trigger which the body's innate immune response receptors sense to begin a cytokine reaction which is toxic to the host.

Application to Biotechnology

Interestingly, enzymes from Ralstonia metallidurans were used by scientists in constructing a fuel cell (3). "By encasing a pair of electrodes coated with these enzymes from Ralstonia metallidurans which oxidize hydrogen put inside a container filled with air and 3 percent more hydrogen" (3). Trials of the fuel cell produced enough electricity to make a watch function and larger scale productions are thought to be possible and are in the making.

Colored scanning electron image of bacterioform gold on a gold grain from the Hit or Miss Mine in northern Queensland. [2]

Current Research

It is being researched that Ralstonia metallidurans could be involved in precipitating gold out of solution. It has the innate ability to thrive in gold concentrations that would kill most other organisms(1). "It is possible that the microbe screens out the gold as part of an effort to detoxify its immediate environment"(1).

References

1.Llyod, Robin. "Eureka! Bacteria Have the Midas Touch" 18 July 2006. http://www.livescience.com/othernews/060718_gold_bacteria.html

2."Ralstonia metallidurans." March 2002. BrookHaven National Labratory. http://genome.bnl.gov/Sequencing/Rmetallidurans/

3.American Chemical Society."New 'biofuel cell' produces electricity from hydrogen in plain air."26 March 2007. http://www.physorg.com/news94144517.html

4. 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

5. Larsen, Rachel and Kit Pogliano. "Outside structures 1: cell wall and membranes." BIMM 120: Introductory Microbiology. University of California, San Diego. Spring 2007. Edited by student of Rachel Larsen and Kit Pogliano