These bacteria turn toxic metals into gold nuggets

A metal-gobbling bacteria manages to ingest toxic metallic compounds and still thrive, producing tiny gold nuggets as a side-effect, scientists have found.

High concentrations of heavy metals, like copper and gold, are toxic for most living creatures, researchers said. However, that is not the case for the bacterium C metallidurans, which has found a way to extract valuable trace elements from a compound of heavy metals without poisoning itself.

The bacteria produces tiny gold nuggets as a side-effect, according to the study published in the journal Metallomics.

A team of researchers from Martin Luther University Halle-Wittenberg (MLU) and the Technical University of Munich (TUM) in Germany has discovered the molecular processes that take place inside the bacteria.

The rod-shaped bacterium C metallidurans primarily lives in soils that are enriched with numerous heavy metals. Over time some minerals break down in the soil and release toxic heavy metals and hydrogen into their environment.

"There is enough hydrogen to conserve energy and nearly no competition," said Professor Dietrich H Nies, a microbiologist at MLU.

"If an organism chooses to survive here, it has to find a way to protect itself from these toxic substances," said Nies.

Together with Professor Frank Reith from the University of Adelaide in Australia, Nies was able to prove in 2009 that C metallidurans is able to deposit gold biologically.

Now, researchers have found that gold enters the bacteria the same way as copper, which is a vital trace element for C metallidurans, however it is toxic in large quantities.

When the copper and gold particles come into contact with the bacteria, a range of chemical processes occur: Copper, which usually occurs in a form that is difficult to be taken up, is converted to a form that is considerably easier for the bacterium to import and thus is able to reach the interior of the cell. The same happens to the gold compounds.

When too much copper has accumulated inside the bacteria, it is normally pumped out by the enzyme CupA, researchers say.

"However, when gold compounds are also present, the enzyme is suppressed and the toxic copper and gold compounds remain inside the cell. Copper and gold combined are actually more toxic than when they appear on their own," said Nies.

To solve this problem, the bacteria activate another enzyme - CopA.

This enzyme transforms the copper and gold compounds into their originally difficult to absorb forms. This assures that fewer copper and gold compounds enter the cellular interior.

"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 nanometres in size," said Nies.

In nature, C metallidurans plays a key role in the formation of so-called secondary gold, which emerges following the breakdown of primary, geologically created, ancient gold ores.

It transforms the toxic gold particles formed by the weathering process into harmless gold particles, thereby producing gold nuggets, researchers said.