Move over, Midas. A genetically modified version of a bacterium that extracts gold from its environment can signal the presence of the precious metal. The result could be a boon for prospectors.

Some bacteria have long been known to be associated with gold deposits, but it has been unclear whether they play a role in its production – and if so, how they do it. Now Frank Reith of the University of Adelaide, South Australia, has found that dissolved gold is harmful to the bacterium Cupriavidus metallic durans, as it forms a toxic, sulphur-containing compound when it is absorbed from the environment. This compound inhi bits the bacterium’s enzyme function, prompting the distressed microbe to activate a cluster of “gold detox” genes that produce enzymes able to convert the soluble gold compounds into harmless particles of metallic gold (Proceedings of the National Academy of Sciences.

Knowing how microbes do this will open up a whole new way of prospecting, says co-author Gregor Grass. He and Reith have developed a genetically modified version of C. metalliduransthat produces a visible response when the detox genes are switched on. “When the microbes come into contact with gold, they flash a l i g ht that can be detected using a hand-held photometer,” says Grass. He envisages that prospectors will be able to detect whether gold is present by taking a sample of soil and adding the modified bacteria to it.