by Gemma Reguera
“The Great Work of the Metal Lover” display, by Brown and Kashefi (Michigan State University) (top image). Included is the aesthetically-engaging, original glass chemostat used to grow the microbe, a metal manifold for gas distribution, and a heated glass copper column that removes any traces of oxygen from the gas supply. Cupriavidus biofilms and the gold grains they produce are visible at the bottom of the vessel (bottom image). Source.
In 2001, Kashefi and collaborators published an article in Applied and Environmental Microbiology reporting the surprising finding that several iron-reducing microbes can use gold as an electron acceptor for their respiration. These microbial alchemists included both mesophilic and thermophilic bacteria as well as hyperthermophilic archaea. The beauty of this process is that the oxidized form of gold provided to the microbes, Au(III), is soluble, whereas its reduced form, Au(0), is insoluble. Hence, the microbes respire soluble gold and precipitate it as gold nanoparticles on their outer surface plus, in the case of the Gram-negative bacteria, in the periplasmic space as well. These studies provided the first experimental evidence supporting the role of microbes in the formation of gold deposits in both hydrothermal and cooler environments, thus challenging the prevailing view that gold mineralization was an abiotic process.
Since then, the list of microbes with the ability to precipitate gold has grown. One of them, the Gram-negative bacterium Cupriavidus metallidurans, is an especially abundant component of the so-called gold nugget microbiota, i.e., the bacterial biofilms associated with gold grains. Elio covered this story previously in our blog. This bacterium grows at room temperature and its mechanism for gold biomineralization is one of the best characterized. For this reason, when artist Brown asked Kashefi, both at Michigan State University, to help him design an exhibit that allowed the audience to witness the process of microbial alchemy, Kashefi regarded Cupriavidus asthe microbe of choice.