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New small molecules protect bacteria against toxic gold


Hamilton, ONMcMaster researchers have discovered that gold resistant bacterium Delftia acidovorans can turn toxic water-soluble gold into a solid gold form, the first demonstration that gold-resistant microbe secretes a metabolite that can protect against toxic gold.

The research, published in Nature Chemical Biology, was led by Nathan Magarvey, an investigator with the Michael G. DeGroote Institute for Infectious Disease Research at McMaster University and Assistant Professor in the Departments of Biochemistry and Biomedical Sciences and Chemistry & Chemical Biology.

“It has long been known that Delftia acidovorans live within gold nuggets, though how the bacteria avoid gold-mediated toxicity has been a mystery,” says Magarvey, adding he and colleagues discovered that a molecule excreted out of the bacteria cells is capable of bringing gold ions together to form large gold aggregates that are no longer toxic.

Magarvey, with his students Chad Johnston, Morgan Wyatt, Ashraf Ibrahim and Xiang Li, and collaborators at Western University, developed a bioinformatics method (published in the Proceedings of the National Academy of Sciences in November 2012) and a natural product tool with Dr. Bin Ma and Lian Yang (University of Waterloo) that assists in the identification of natural products, and assisted in identifying the peptide-like delftibactin A small molecules. Using this method, and biological activity assays, the team grew a colony of the bacterium Delftia acidovorans, and conducted tests to determine how it produces the molecular-sized gold nuggets outside its cell wall. They concluded the answer lies in a part to a molecule excreted by the microbe that both shields the organism and transforms the poisonous ions into particles.

“Our paper details a novel mechanism by which bacteria found on gold may protect themselves from toxic heavy metals, in this case gold,” says Johnston. “There is a protective mechanism by the gold resin bacteria in that it secretes a small molecule that binds it. What is interesting about this is the bacterium precipitates the molecule and forms gold nanoparticles and tiny gold platelets that are essentially the seeds of gold nuggets resembling those you would find in natural geological deposits. It is a novel mechanism for gold biomineralization, an extremely understudied but emerging field.”

“The natural function of molecules is a very fascinating subject and sometimes you can relate that to the organism based on some environmental context,” adds Magarvey. “By isolating and characterizing these molecules, we revealed the molecular mechanisms of how that thing works with respect to gold ion complexation. That gives insight into the types of molecules and the types of strategies that these gold associated bacteria may be using to deposit gold. Whether that was exactly what happens in nature we’re not sure but at least this gives us some insight. Moving down the line maybe there are ways we can harness this toward ideas and concepts that would help “greener” extraction methodologies. That’s not our primary focus but there is the possibility that maybe that can be done.”

Magarvey hopes this finding will open doors to understanding the importance of natural small molecules in the environment and also within medicine.  “Our focus is on how bacteria make molecules and the benefits that these molecules may have in medicine and in their ecological settings. Our next targets are those bugs that live on or within us and realizing what roles human-associated bacteria and their metabolites play in our well-being and health.”

Reported by Chantall Van Raay, McMaster University