Alchemists are known to possess power to turn base metals into noble metals like gold. In a recent study a team led by Nathan Magarvey at McMaster University in Hamilton, Ontario, reported a type of bacteria that can produce gold particles when exposed to gold solution. The results of this interesting study has been published as a brief communication in Advanced Online publication section of February, 2013 issue of Nature Chemical Biology under the title “Gold biomineralization by a metallophore from a gold-associated microbe”. They found this gold resident bacterium Delftia acidovorans secretes a secondary metabolite, later named ‘delftibactin’ that can protect it against toxic liquid form of gold causing gold biomineralization.
To understand the mechanism of this biomieralization the authors compared it with another gold resident bacteria C. metallidurans which bioaccumulates inert gold nanoparticles within its cytoplasm. But D. acidivorans does this extracellularly which was revealed by a simple test in culture plate with liquid gold where D acidivorans created a black zone indicating reduction reaction of gold and formation of solid gold particles extracellularly. The researchers then compared the genomes of C. metallodurans and D. acidivorans with a view to identifying some unique small molecule biosynthesis genes that is absent in C. metallidurans but present in D. acidivorans. This characteristic may be responsible for the formation of solid gold particles outside the membrane in D. acidivorans. Their analysis identified a candidate non-ribosomal peptide synthetise/polyketide synthase (NRPS/PKS) gene cluster and named ‘del’ cluster for an unknown secondary metabolite. Insertional inactivation of one of the genes in this cluster proved that the genes are involved in gold biomineralization. Metabolome study from the mutant and wild type could identify the role of the synthesized protein and was named ‘delftibactin’. They proposed delftibactin facilitates the biomineralization thereby protecting D. acidivorans by chelating soluble gold and precipitating it as a complex or by binding and reducing gold through oxidative decarboxylation. Further study revealed that delftibactin is most likely a siderophore which also chelates iron, thus being more advantageous to the bacteria to function under toxic conditions. The group claims this discovery of delftibactin as the first example of a co-opted metallophore that protects the product from toxic soluble gold and provides a mechanism for bacterial gold biomineralization. [summarized by the graduate student of Plant Biotech Lab., DU]