Like many women, this bug too loves to be surrounded by gold. But life is not all shiny for the bacterium Delftia acidovorans as it has to fight off a lethal effect that an active form of yellow metal has on it. The survival strategy that the microbe has evolved over millions of years may now offer a novel way to discover new gold deposits, if not a new means of mining gold.
This week, a team of researchers led by biochemist Nathan Magarvey of Canada’s McMaster University reported in the journal Nature Chemical Biology that the microbe produces a chemical that helps it convert toxic gold ions into inert gold particles so that it can live within gold nuggets found in nature.
A large number of microorganisms feed on metal ions present in the environment because these metals are essential to meet their energy and nutrition requirements. (One of the most sought after elements by bacteria is iron, which is important for their growth.) In the process, the bugs play a part in the formation of different mineral deposits, including in some of the harshest conditions that exist on earth. But, until very recently, nobody thought that microbes could also survive on a rare and inert metal like gold. As a result, many argued that microbes had little role in the formation of gold minerals, which miners tap to make the precious metal.
In 2006, a team of scientists led by Frank Reith of Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO) busted this theory by providing strong evidence that bacteria contribute directly to the formation of gold nuggets. Subsequent sequencing by them and groups elsewhere showed two types of bacteria — D. acidovorans and Cupriavidus metallidurans — that thrive on gold.
But how these bugs survive a toxic effect that gold ions have on them remained a mystery till Reith’s team resolved the case of C. metallidurans in 2009. It found that the bacteria have an internal mechanism that helps them convert the toxic metal ions into innocuous gold particles. However, the mechanism employed by D. acidovorans continued to be an enigma as it was patently different from that of C. metallidurans.
“This is an excellent study which shows how different organisms living in biofilms on gold grains have different strategies of dealing with gold toxicity,” says Reith.
Unlike C. metallidurans which takes in toxic gold ions and excretes inert gold particles, D. acidovorans cells release a chemical (peptide) which brings together gold ions to form large gold aggregates that are no longer toxic to the bugs. Subsequently, the Canadian scientists discovered that the natural chemical that the microbe uses has an active molecule called delftibactin A. Through further studies, the team showed that the compound is similar to what scientists know as siderophore — a class of molecules that bind to iron — and that D. acidovorans co-opted this common molecule to eliminate the gold toxicity that it cannot withstand.
The new finding by Magarvey and his colleagues not only indicates the versatility in strategies that microbes employ to deal with the same kind of problem, but also provides an excellent biological sensor that can be used to prospect for gold. The scientist, however, is nonchalant about the potential applications of their study. “We are an academic group which focuses on natural processes, not industrial ones. Potential industrial or commercial applications are not the original focus of the study,” he said. The next step of his team will be to look at molecules of this type which help create different nanoparticle sizes and solid gold shapes.
Reith, on the other hands, thinks it is indeed useful as a biosensor for gold. “If we can show that delftibactin is selective for gold (does not bind to other metals in a similar quantity), it might even be useful for gold recovery,” says the CSIRO scientist, quickly adding that more research is needed to understand the interaction of delftibactin with metals in environmental situations.