Mythical King Midas was ultimately doomed because everything he touched turned to gold. Now, the reverse has been found in bacteria that owe their survival to a natural Midas touch.
Delftia acidovorans lives in sticky biofilms that form on top of gold deposits, but exposure to dissolved gold ions can kill it. That's because although metallic gold is unreactive, the ions are toxic.
To protect itself, the bacterium has evolved a chemical that detoxifies gold ions by turning them into harmless gold nanoparticles. These accumulate safely outside the bacterial cells.
"This could have potential for gold extraction," says Nathan Magarvey of McMaster University in Hamilton, Ontario, who led the team that uncovered the bugs' protective trick. "You could use the bug, or the molecules they secrete."
He says the discovery could be used to dissolve gold out of water carrying it, or to design sensors that would identify gold-rich streams and rivers.
The protective chemical is a protein dubbed delftibactin A. The bugs secrete it into the surroundings when they sense gold ions, and it chemically changes the ions into particles of gold 25 to 50 nanometres across. The particles accumulate wherever the bugs grow, creating patches of gold.
Deep purple gold
But don't go scanning streams for golden shimmers: the nanoparticle patches do not reflect light in the same way as bigger chunks of the metal – giving them a deep purple colour.
When Magarvey deliberately snipped out the gene that makes delftibactin A, the bacteria died or struggled to survive exposure to gold chloride. Adding the protein to the petri dish rescued them.
The bacterium Magarvey investigated is one of two species that thrive on gold, both identified a decade or so ago by Frank Reith of the University of Adelaide in Australia. In 2009 Reith discovered that the other species, Cupriavidus metallidurans, survives using the slightly riskier strategy of changing gold ions into gold inside its cells.
"If delftibactin is selective for gold, it might be useful for gold recovery or as a biosensor," says Reith. "But how much dissolved gold is out there is difficult to say."
Journal reference: Nature Chemical Biology, DOI: 10.1038/NCHEMBIO.1179
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