If the goose that laid the golden egg had a real-life counterpart, it would be C. metallidurans. This hardy little bacterium consumes toxic metals and excretes tiny gold nuggets, but how and why it does so has never been fully understood. Now, German and Australian researchers have peered inside the microorganism and figured out that mechanism.

C. metallidurans has carved out a nice little niche for itself, usually living in soils full of heavy metals, which are toxic to most other microorganisms. But this bacteria has evolved a defense mechanism to help it not only survive but thrive under those conditions, and its ability to turn toxic compounds into gold is well known enough to once earn it a place in an alchemy art installation.

"Apart from the toxic heavy metals, living conditions in these soils are not bad," says Dietrich H. Nies, an author on the new study. "There is enough hydrogen to conserve energy and nearly no competition. If an organism chooses to survive here, it has to find a way to protect itself from these toxic substances."

The researchers, from Martin Luther University Halle-Wittenberg (MLU), the Technical University of Munich (TUM) and the University of Adelaide, have now identified just how it does that. C. metallidurans needs copper to live, and to extract that trace element from its surroundings the bacteria converts it into a form that's easier to "import."

But there are a few problems. High levels of copper are toxic, and that's not the only heavy metal in the soil. Gold compounds are also imported into its body through the same process. Not only is this natural form of gold pretty toxic, but it's even more potent when mixed with copper.

To deal with the first problem, C. metallidurans has an enzyme called CupA that pumps out excess copper. When gold and copper are both taken up, the bacteria switches off this enzyme and fires up a different one, CopA. This new enzyme converts the gold and copper back into their original, hard-to-digest forms, effectively solving the second problem.

"This assures that fewer copper and gold compounds enter the cellular interior," says Nies. "The bacterium is poisoned less and the enzyme that pumps out the copper can dispose of the excess copper unimpeded. Another consequence: the gold compounds that are difficult to absorb transform in the outer area of the cell into harmless gold nuggets only a few nanometers in size."

Through this process, C. metallidurans may be responsible for some of the Earth's natural "secondary gold." Primary gold refers to the ancient deposits formed geologically, while secondary gold is much younger, closer to the surface and often found in nugget form. This is often the result of groundwater dissolving primary gold and transporting it upwards, but certain bacteria may also "chew" off tiny flakes of primary gold that then move towards the surface. There, C. metallidurans could convert it into secondary gold, ready for a lucky prospector.

The researchers say that further understanding of this cycle may eventually allow gold to be wrung out of less rich ores, without the need for toxic chemicals like mercury.

The research was published in the journal Applied and Environmental Microbiology.

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