The biochemical fate of Ag+ ions in Staphylococcus aureus, Escherichia coli, and biological media.

Silver is commonly included in a range of household and medical items to provide bactericidal action. Despite this, the chemical fate of the metal in both mammalian and bacterial systems remains poorly understood. Here, we applied a metallomics approach using X-ray absorption spectroscopy (XAS) and size-exclusion chromatography hyphenated with inductively coupled plasma mass spectrometry (SEC-ICP-MS) to advance our understanding of the biochemical fate of silver ions in bacterial culture and cells, and the chemistry associated with these interactions. When silver ions were added to lysogeny broth, silver was exclusively associated with moderately-sized species (~30 kDa) and bound by thiolate ligands. In two representative bacterial pathogens cultured in lysogeny broth including sub-lethal concentrations of ionic silver, silver was found in cells to be predominantly coordinated by thiolate species. The silver biomacromolecule-binding profile in Staphylococcus aureus and Escherichia coli was complex, with silver bound by a range of species spanning from 20 kDa to >1220 kDa. In bacterial cells, silver was nonuniformly colocalised with copper-bound proteins, suggesting that cellular copper processing may, in part, confuse silver for nutrient copper. Notably, in the treated cells, silver was not detected bound to low molecular weight compounds such as glutathione or bacillithiol.

Silver in biology and medicine: opportunities for metallomics researchers.

The antibacterial properties of silver have been known for centuries and the threat of antibiotic-resistant bacteria has led to renewed focus on the noble metal. Silver is now commonly included in a range of household and medical items to imbue them with bactericidal properties. Despite this, the chemical fate of the metal in biological systems is poorly understood. Silver(I) is a soft metal with high affinity for soft donor atoms and displays much similarity to the chemistry of Cu(I). In bacteria, interaction of silver with the cell wall/membrane, DNA, and proteins and enzymes can lead to cell death. Additionally, the intracellular generation of reactive oxygen species by silver is posited to be a significant antimicrobial action. While the antibacterial action of silver is well known, bacteria found in silver mines display resistance against it through use of a protein ensemble thought to have been specifically developed for the metal, highlighting the need for judicious use. In mammals, ∼10-20% of ingested silver is retained by the body and thought to predominantly localize in the liver or kidneys. Chronic exposure can result in argyria, a condition characterized by blue staining of the skin, resulting from subdermal deposition of silver [as Ag(0)/sulfides], but more insidious side effects, such as inclusions in the brain, seizures, liver/kidney damage, and immunosuppression, have also been reported. Here, we hope to highlight the current understanding of the biological chemistry of silver and the necessity for continued study of these systems to fill existing gaps in knowledge.

A Stable Coordination Polymer Based on Rod-Like Silver(I) Nodes with Contiguous Ag-S Bonding.

Silver(I)-based coordination polymers or metal-organic frameworks (MOFs) display useful antibacterial properties, whereby distinct materials with different bonding can afford control over the release of silver(I) ions. Such silver(I) materials are comprised of discrete secondary building units (SBUs), and typically formed with ligands possessing only soft or borderline donors. We postulated that a linker with four potential donor groups, comprising carboxylate and soft thioether donors, 2,5-bis (allylsulfanyl) benzene dicarboxylic acid (ASBDC), could be used to form stable, highly connected coordination polymers with silver(I). Here, we describe the synthesis of a new material, (Ag2(ASBDC)), which possesses a rod-like metal node-based 3D honeycomb structure, strongly -stacked linkers, and steric bulk to protect the node. Due to the rod-like metal node and the blocking afforded by the ordered allyl groups, the material displays notable thermal and moisture stability. An interesting structural feature of (Ag2(ASBDC)) is contiguous Ag-S bonding, essentially a helical silver chalcogenide wire, which extends through the structure. These interesting structural features, coupled with the relative ease by which MOFs made with linear dicarboxylate linkers can be reticulated, suggests this may be a structure type worthy of further investigation.