Scientists say they have engineered bacteria that can mop up mercury, a step forward in the goal of using "bioremediation" to cleanse toxic chemicals from the environment.

Mercury pollution of water and soil by industrial spills or gold mining is a major hazard because the chemical accumulates up the food chain, and tackling the problem is prohibitively expensive.

A team led by Oscar Ruiz of the Inter-American University of Puerto Rico say they inserted two mercury-friendly genes into the common intestinal germ, Escherichia coli.

The genes controlled a protein called metallothionein and an enzyme known as polyphosphate kinase, both of which bind to metals in order to prevent toxicity in cells.

The researchers immersed the engineered bugs in a nutrient solution to which high doses of mercury had been added.

The E. coli with genes for polyphosphate kinase grew in mercury levels that were 16 times higher than the maximum for normal strains of the bug.

Even more impressive, those with the genes for metallothionein flourished in 24 times the maximum mercury level.

After five days, the germs were spun in a centrifuge. Chemical analysis showed the bacteria had absorbed up to 80 percent of the mercury to which they had been exposed.

The study appears in the latest issue of the open-access, peer-reviewed journal BMC Biotechnology.

In an email exchange with AFP, Ruiz said the experiments opened the way to a viable tool to not only clean up mercury but also recycle it, using electrochemistry to extract the chemical from the bacteria.

"Once [mercury] pollutes a body of water it is very difficult to clean, due to the intense process of removing and transporting large volumes of water to an external location, then treating the contaminated water, and finally disposing of the treated water," said Ruiz.

"In the approach we envision, the water can be pumped in situ into columns containing the engineered bacteria and then, once cleaned, it can be returned to the site, causing minimal disruption of the ecosystem."

The scientists have not calculated the cost of using their technique in the field, but a back-of-the-envelope estimate is that it would beat current clean-up technologies, which cost $100,000-$150,000 per kilo ($40,000-$70,000 per pound) of mercury, he suggested.

Genetically modified organisms are viewed with concern by environmentalists, who fear their DNA could mingle with other germs or lifeforms, creating a peril for human health or biodiversity.

Ruiz said the bacteria used in the study was a "harmless, non-pathogenic" type whose genetic modifications presented no environmental threat and, in a clean-up, would be used in enclosed containers.

He added that, in any case, new GMOs had to be vetted by safety watchdogs in the United States, as in other countries.

The technique has only been tested on removing mercury from liquids, not from soils, for which further work would be needed, said Ruiz.

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