Thursday, July 30, 2015

A tough bacterium that lives in poisoned soils and pulls gold out of water

Cupriavidus metallidurans (roughly translated: lover of copper, enduring metal) is a bacterium of the class Betaproteobacteria known for its ability to withstand high concentrations of numerous metals that would be toxic to most other living things. These metals, which include Ag, Au, Bi, Cd, Co, Cr, Cs, Cu, Hg, Ni, Pb, Sr, Tl, U, and Zn, tend to cause problems for bacterial cells by binding to DNA or proteins, which can disrupt important stuff like obtaining energy or reproducing.

The bacterium has in its possession a large set of chromosome- and plasmid-based metal resistance genes that encode various efflux systems (e.g. P-type ATPases). These work to minimize the adverse impact of metal ions, which sneak inside cells on a regular basis, by pumping them out. After being evicted, exterior components of the bacterial cell wall can capture the metal ions and keep them from coming back inside. The extensive genetic resistance repertoire possessed by C. metallidurans is due in part to its propensity to obtain plasmids from other bacteria via horizontal gene transfer.

Reflecting its metal toughness, C. metallidurans is often a major member of microbial communities in environments enriched with metals, be they natural or created by neglectful human activities. For example, the bacterium was originally isolated from a metal-rich sludge left over from processing zinc at a factory in Belgium. Other potential hangouts include soils and sediments at or near mining and smelting sites, metal plating factories, tanneries, and orchards sprayed with metal-containing pesticides.

Probably not a bad place to look for Cupriavidus metallidurans

C. metallidurans is a survivor. Not only can it tolerate high metal concentrations, but it's a facultative chemolithoautotrophic, facultative anaerobe. That's a fancy way of saying it can live with or without oxygen and can make it's own parts by pulling carbon dioxide out of the air and fusing it together using energy derived from sulfur or hydrogen gas. It's also happy to use organic carbon, if it's available, including aromatics such as toluene and phenol that are widespread pollutants of soil and groundwater. Among the harsher environments the bacterium has been isolated from are pools of depleted nuclear fuel and ultra-clean rooms in which spacecraft are put together prior to launch. It's also been found hanging out in the water systems of the International Space Station.

Due to its resilience, C. metallidurans has been investigated for its ability to grow on and leach nutrients from basalt, an igneous rock that is a major component of regolith on Mars and the Moon. The idea is the bacterium could be used to generate resources from a material already present in space, assisting colonization efforts.

Relative to chemical techniques, the removal of metals from contaminated environments by bacteria can be cheap, simple, and low impact. C. metallidurans has been investigated as a means of cleaning up sites with harsh conditions not suitable for other microbes (e.g. there not being much organic matter to eat). One approach is to add the bacterium to a bunch of wet sand and let it form biofilms around individual sand particles. Wastewater full of metal ions is then passed through the sand and the bacterium acts to pull the metals out of the water and trap them in the sand/bacteria mix. Alternatively, C. metallidurans can be mixed with water and metal-contaminated soil in a reactor. The bacterium likes to float to the surface as it grows, providing a means to separate metals (bound to its cells) from soils that settle to the bottom.

In an effort to improve the relatively low mercury resistance of C. metallidurans (compared to the other metals it resists) and thus improve its utility in cleaning up mercury-contaminated environments (which are often laced with other metals), researchers managed to gift it with a plasmid containing several mercury resistance genes. This plasmid had been previously isolated from a microbial community residing in mercury-polluted river sediments, and enables the bacterium to remove mercury cations from polluted water by giving them electrons and thereby increasing their volatility.

With its new plasmid, the modified bacterium (MSR33) can remove mercury from water (Source)

The bacterium has also been genetically engineered to create a tool for detecting metals in soils and other materials. A chunk of DNA from a bioluminescent bacterium called Aliivibrio fischeri was inserted alongside genes encoding resistance to particular metals. In the presence of these metals (at a certain minimum concentration), the modified bacterium will express both metal resistance proteins and luciferase, a light-producing enzyme. The intensity of the light can be used to measure metal concentrations.

Being awesome, C. metallidurans contributes to the formation of placer gold deposits via the conversion of soluble gold chloride complexes into solid elemental gold particles. Weathering or mining of gold-containing rock can lead to the formation of soluble gold complexes that enter nearby natural waters. Cells of the bacterium are able to fill themselves up with these complexes, which they then transform (via the addition of electrons) into gold-carbon compounds and subsequently into nanoparticles of elemental gold. This appears to be a detoxification strategy since gold complexes can adversely affect cells, the side effect being the precipitation of gold out of natural waters as grains (which can be separated out from sediments by panning or sluicing).

One of the more interesting applications of the bacterium's gold precipitating ability is in assessing the utility of human hair as a measure of metal exposure prior to death in buried corpses. Investigators found that C. metallidurans was able to deposit gold in human hair that had been buried in soil for six months. Since gold is fairly stable in the environment, this finding suggests soil bacteria can also influence the concentrations of other metals in buried hair. So anyone who wants to use the metal content of this hair to infer stuff about the lives of the person who grew the hair (e.g. archaeologists, forensic investigators) probably needs to consider the activity of soil bacteria and how long the hair has been buried.

Tiny clumps of Cupriavidus metallidurans and gold on a strand of hair (Source)

On the medical side of things, C. metallidurans is very rarely able to get inside people and cause disease. In the one case I looked at, the person affected was already very sick with diabetes and heart disease, and had just recently had intensive abdominal surgery to remove several cancerous parts. The bacterium has also been recovered from the respiratory tract of cystic fibrosis (CF) patients. It's not clear if it actually infects these folks and contributes to their illness or if it just is hanging out without causing problems (apparently unexpected bacteria tend to show up in CF-impacted lungs).


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