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

The good, the bad, and the ugly of metals as antimicrobials

We are now moving into the antimicrobial resistance (AMR) era where more antibiotic resistant bacteria are now the majority, a problem brought on by both misuse and over use of antibiotics. Unfortunately, the antibiotic development pipeline dwindled away over the past decades as they are not very profitable compounds for companies to develop. Regardless researchers over the past decade have made strides to explore alternative options and out of this we see revisiting historical infection control agents such as toxic metals. From this we now see a field of research exploring the efficacy of metal ions and metal complexes as antimicrobials. Such antimicrobials are delivered in a variety of forms from metal salts, alloys, metal complexes, organometallic compounds, and metal based nanomaterials and gives us the broad term metalloantimicrobials. We now see many effective formulations applied for various applications using metals as antimicrobials that are effective against drug resistant strains. The purpose of the document here is to step aside and begin a conversation on the issues of use of such toxic metal compounds against microbes. This critical opinion mini-review in no way aims to be comprehensive. The goal here is to understand the benefits of metalloantimicrobials, but also to consider strongly the disadvantages of using metals, and what are the potential consequences of misuse and overuse. We need to be conscious of the issues, to see the entire system and affect through a OneHealth vision.

Silver Organometallics that are Highly Potent Thioredoxin and Glutathione Reductase Inhibitors: Exploring the Correlations of Solution Chemistry with the Strong Antibacterial Effects

The antibacterial activity of silver species is well-established; however, their mechanism of action has not been adequately explored. Furthermore, issues of low-molecular silver compounds with cytotoxicity, stability, and solubility hamper their progress to drug leads. We have investigated silver N-heterocyclic carbene (NHC) halido complexes [(NHC)AgX, X = Cl, Br, and I] as a promising new type of antibacterial silver organometallics. Spectroscopic studies and conductometry established a higher stability for the complexes with iodide ligands, and nephelometry indicated that the complexes could be administered in solutions with physiological chloride levels. The complexes showed a broad spectrum of strong activity against pathogenic Gram-negative bacteria. However, there was no significant activity against Gram-positive strains. Further studies clarified that tryptone and yeast extract, as components of the culture media, were responsible for this lack of activity. The reduction of biofilm formation and a strong inhibition of both glutathione and thioredoxin reductases with IC50 values in the nanomolar range were confirmed for selected compounds. In addition to their improved physicochemical properties, the compounds with iodide ligands did not display cytotoxic effects, unlike the other silver complexes. In summary, silver NHC complexes with iodide secondary ligands represent a useful scaffold for nontoxic silver organometallics with improved physicochemical properties and a distinct mechanism of action that is based on inhibition of thioredoxin and glutathione reductases.

Recent Advances in Metalloproteomics

Interactions between proteins and metal ions and their complexes are important in many areas of the life sciences, including physiology, medicine, and toxicology. Despite the involvement of essential elements in all major processes necessary for sustaining life, metalloproteomes remain ill-defined. This is not only owing to the complexity of metalloproteomes, but also to the non-covalent character of the complexes that most essential metals form, which complicates analysis. Similar issues may also be encountered for some toxic metals. The review discusses recently developed approaches and current challenges for the study of interactions involving entire (sub-)proteomes with such labile metal ions. In the second part, transition metals from the fourth and fifth periods are examined, most of which are xenobiotic and also tend to form more stable and/or inert complexes. A large research area in this respect concerns metallodrug-protein interactions. Particular attention is paid to separation approaches, as these need to be adapted to the reactivity of the metal under consideration.

Cu+/Ag+ Competition in Type I Copper Proteins (T1Cu)

Due to the similarity in the basic coordination behavior of their mono-charged cations, silver biochemistry is known to be linked to that of copper in biological systems. Still, Cu⁺/²⁺ is an essential micronutrient in many organisms, while no known biological process requires silver. In human cells, copper regulation and trafficking is strictly controlled by complex systems including many cytosolic copper chaperones, whereas some bacteria exploit the so-called "blue copper" proteins. Therefore, evaluating the controlling factors of the competition between these two metal cations is of enormous interest. By employing the tools of computational chemistry, we aim to delineate the extent to which Ag⁺ might be able to compete with the endogenous copper in its Type I (T1Cu) proteins, and where and if, alternatively, it is handled uniquely. The effect of the surrounding media (dielectric constant) and the type, number, and composition of amino acid residues are taken into account when modelling the reactions in the present study. The obtained results clearly indicate the susceptibility of the T1Cu proteins to a silver attack due to the favorable composition and geometry of the metal-binding centers, along with the similarity between the Ag⁺/Cu⁺-containing structures. Furthermore, by exploring intriguing questions of both metals' coordination chemistry, an important background for understanding the metabolism and biotransformation of silver in organisms is provided.

Zn(II) to Ag(I) Swap in Rad50 Zinc Hook Domain Leads to Interprotein Complex Disruption through the Formation of Highly Stable Agx(Cys)y Cores

The widespread application of silver nanoparticles in medicinal and daily life products increases the exposure to Ag(I) of thiol-rich biological environments, which help control the cellular metallome. A displacement of native metal cofactors from their cognate protein sites is a known phenomenon for carcinogenic and otherwise toxic metal ions. Here, we examined the interaction of Ag(I) with the peptide model of the interprotein zinc hook (Hk) domain of Rad50 protein from Pyrococcus furiosus, a key player in DNA double-strand break (DSB) repair. The binding of Ag(I) to 14 and 45 amino acid long peptide models of apo- and Zn(Hk)₂ was experimentally investigated by UV-vis spectroscopy, circular dichroism, isothermal titration calorimetry, and mass spectrometry. The Ag(I) binding to the Hk domain was found to disrupt its structure via the replacement of the structural Zn(II) ion by multinuclear Ag_x(Cys)_y complexes. The ITC analysis indicated that the formed Ag(I)-Hk species are at least 5 orders of magnitude stronger than the otherwise extremely stable native Zn(Hk)₂ domain. These results show that Ag(I) ions may easily disrupt the interprotein zinc binding sites as an element of silver toxicity at the cellular level.

Comparison of Antimicrobial and Antibiofilm Activity of Proflavine Co-crystallized with Silver, Copper, Zinc, and Gallium Salts

Here, we exploit our mechanochemical synthesis for co-crystallization of an organic antiseptic, proflavine, with metal-based antimicrobials (silver, copper, zinc, and gallium). Our previous studies have looked for general antimicrobial activity for the co-crystals: proflavine·AgNO3, proflavine·CuCl, ZnCl3[Proflavinium], [Proflavinium]2[ZnCl4]·H2O, and [Proflavinium]3[Ga(oxalate)3]·4H2O. Here, we explore and compare more precisely the bacteriostatic (minimal inhibitory concentrations) and antibiofilm (prevention of cell attachment and propagation) activities of the co-crystals. For this, we choose three prominent "ESKAPE" bacterial pathogens of Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus. The antimicrobial behavior of the co-crystals was compared to that of the separate components of the polycrystalline samples to ascertain whether the proflavine-metal complex association in the solid state provided effective antimicrobial performance. We were particularly interested to see if the co-crystals were effective in preventing bacteria from initiating and propagating the biofilm mode of growth, as this growth form provides high antimicrobial resistance properties. We found that for the planktonic lifestyle of growth of the three bacterial strains, different co-crystal formulations gave selectivity for best performance. For the biofilm state of growth, we see that the silver proflavine co-crystal has the best overall antibiofilm activity against all three organisms. However, other proflavine-metal co-crystals also show practical antimicrobial efficacy against E. coli and S. aureus. While not all proflavine-metal co-crystals demonstrated enhanced antimicrobial efficacy over their constituents alone, all possessed acceptable antimicrobial properties while trapped in the co-crystal form. We also demonstrate that the metal-proflavine crystals retain antimicrobial activity in storage. This work defines that co-crystallization of metal compounds and organic antimicrobials has a potential role in the quest for antimicrobials/antiseptics in the defense against bacteria in our antimicrobial resistance era.

Structures of Silver Fingers and a Pathway to Their Genotoxicity

Recently, we demonstrated that Ag^I can directly replace Zn^II in zinc fingers (ZFs). The cooperative binding of Ag^I to ZFs leads to a thermodynamically irreversible formation of silver clusters destroying the native ZF structure. Thus, a reported loss of biological function of ZF proteins is a likely consequence of such replacement. Here, we report an X-ray absorption spectroscopy (XAS) study of Ag_n S_n clusters formed in ZFs to probe their structural features. Selective probing of the local environment around Ag^I by XAS showed the predominance of digonal Ag^I coordination to two sulfur donors, coordinated with an average Ag-S distance at 2.41 Å. No Ag-N bonds were present. A mixed AgS₂ /AgS₃ geometry was found solely in the CCCH Ag^I -ZF. We also show that cooperative replacement of Zn^II ions with the studied Ag₂ S₂ clusters occurred in a three-ZF transcription factor protein 1MEY#, leading to a dissociation of 1MEY# from the complex with its cognate DNA.

Using a chemical genetic screen to enhance our understanding of the antimicrobial properties of copper

The competitive toxic and stress inducing nature of copper necessitates systems that sequester and export this metal from the cytoplasm of bacterial cells. Several predicted mechanisms of toxicity include the production of reactive oxygen species, thiol depletion, DNA and iron-sulfur cluster disruption. Accompanying these mechanisms include pathways of homeostasis such as chelation, oxidation, and transport. Still, the mechanisms of copper resistance and sensitivity are not fully understood. Furthermore, studies fail to recognize that the response to copper is likely a result of numerous mechanisms, as in the case for homeostasis, in which proteins and enzymes work as a collective to maintain appropriate copper concentrations. In this study we used the Keio collection, an array of 3985 Escherichia coli mutants, each with a deleted non-essential gene, to gain a better understanding of prolonged copper exposure. In short, we recovered two copper homeostatic gene and genes involved in transporting and assembling to be involved in mediating prolonged copper stress under the conditions assessed. The gene coding for the protein TolC was uncovered as a sensitive hit and we demonstrated that tolC, an outer membrane efflux channel, is key in mitigating copper sensitivity. Additionally, the activity of tRNA processing was enriched and the deletion of several proteins involved in import generated copper tolerance. Lastly, key genes belonging to central carbon metabolism and nicotinamide adenine dinucleotide biosynthesis were uncovered as tolerant hits. Overall, this study shows that copper sensitivity and tolerance are a result of numerous mechanisms acting in combination within the cell.