High concentrations of heavy metals, like copper and gold, are toxic for most living creatures. This 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. One interesting side-effect: the formation of tiny gold nuggets.This bacteria ordinarily lives in soils that are laced with copper and toxic metals. Over time, some minerals break down in the soil and release these heavy metals and hydrogen (acid) into their environment.
“Apart from the toxic heavy metals, living conditions in these soils are not bad. 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,” explains Professor Dietrich H. Nies, a microbiologist at MLU.MLU is Martin Luther University Halle-Wittenberg in Germany. The other co-authors were at the Technical University of Munich (TUM) and the University of Adelaide in Australia. Professor Nies and his Australian counterpart, Professor Frank Reith, concluded that C. metallidurans is able to deposit gold biologically back in 2009 but only now have elucidated how they do it.
Gold enters the bacteria the same way as copper. Copper 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 also happens to the gold compounds.
When too much copper has accumulated inside the bacteria, it is normally pumped out by the enzyme CupA. “However, when gold compounds are also present, the enzyme is supressed 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,” says Dietrich H. 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,” says Nies.