An investigation of the chemistry of silver in biological systems, and the development of silver-containing materials for use as antibacterial agents

Abstract

The antibacterial properties of silver have been known for centuries; however, 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 mammalian or bacterial biological systems, is poorly understood. Through a metallomics approach, using techniques like X-ray absorption spectroscopy (XAS), and size-exclusion chromatography hyphenated inductively coupled plasma mass spectrometry (SEC-ICP-MS), results are presented that advance our understanding of the chemistry of these interactions. In bacterial systems, endogenous and exogenous silver ions were found to be predominantly coordinated by thiolate species, likely cysteine residues. Silver in broth was found to be associated exclusively with moderately size species (~30 kDa). Conversely, in S. aureus and E. coli, silver was bound by a range of species, ranging from 20-30 kDa, to >1220 kDa. Silver was also nonuniformly associated with copper-bound species, suggesting possible cellular processing of the noble metal as nutrient copper. Colocation of Ag with Cu could also provide insight into the cellular distribution of silver, as copper is predominantly found in the extracytoplasmic space of bacteria as it is a strictly regulated nutrient due to potential toxicity. Exposure of human whole blood to 2 mM AgNO3 resulted in lysis of red blood cells (RBCs), regardless of the isotonicity of the added solution. Despite this, within five minutes ~90% of silver ions were localised in the RBCs. Inside the RBCs Ag+ interacted predominantly with haemoglobin and no low molecular weight complexes (e.g. glutathione). Linear combination fitting of Ag K-edge XANES of the RBC fraction from a sample of human whole blood treated with AgNO3 indicated ~53% of the experimental spectrum could be described by a silver-haemoglobin model with the remaining percent explained by a silver-cysteine model. In isolated plasma the speciation of silver was found to strongly resemble that of solid silver chloride. Conversely, the speciation of silver in the plasma fraction from whole blood was found to have no contribution from the same AgCl model spectrum. The dominant binder of silver in human plasma was human serum albumin as well as higher molecular weight species. Finally, the pore environment of UiO-66 was tailored for use as an antibacterial agent. In. general, inclusion of sulfur-containing functionality in the framework resulted in increased silver ion uptake (relative to the parent material). While sulfhydryl-decorated pores were found to take up significantly more silver, the material did not release significant concentrations of Ag+. Allyl and alkyl thioethers were found to both increase silver uptake and control release. The silver-loaded materials were successfully embedded into polymer matrices; extending applicability as antibacterial hybrid coatings and providing further control over silver release. Ag XAS revealed that, when loaded with silver ions, the unfunctionalised UiO-66 partially reduced Ag(I) to Ag(0) in the absence of an added reductant. All silver-loaded UiO-66 derivatives embedded in polymer matrices were found to have antibacterial activity against S. aureus and E. coli.