Heavy metals are encountered in nature, and are used in several human endeavors, including in dental fillings. It is well known that the safety of metals depends on their chemical form, as well as the dose and route through which biological systems are exposed to them. Here, we used the Nauphoeta cinerea model to examine the mechanism by which salts of the heavy metals used in dental fillings - silver and mercury - exert their neurotoxicity. Nymphs exposed to heavy metals presented with reduced motor and exploratory abilities as they spent more time immobile, especially in the periphery of a novel object, and covered less distance compared with control nymphs. Exposure to AgNO3 and HgCl2 also exacerbated levels of oxidative stress markers (MDA & ROS) and the neurotransmitter regulators - AChE and MAO, while reducing antioxidant activity markers, both in biochemical (thiol & GST) and RT-qPCR (TRX, GST, SOD, Catalase) examinations, in neural tissues of the cockroach. The observed disruptions in neurolocomotor control, synaptic transmission and redox balance explain how heavy metal salts may predispose organisms to neurological disorders.

The mechanisms of toxicity of engineered nanomaterials (ENMs) to the early life stages of freshwater fish, and the relative hazard compared to dissolved metals, is only partially understood. In the present study, zebrafish (Danio rerio) embryos were exposed to lethal concentrations of silver nitrate (AgNO3) or silver (Ag) ENMs (primary size 42.5 ± 10.2 nm). The 96 h-LC50 for AgNO3 was 32.8 ± 0.72 μg Ag L-1 (mean ± 95% CI) compared to 6.5 ± 0.4 mg L-1 of the whole material for Ag ENMs; with the ENMs being orders of magnitude less toxic than the metal salt. The EC50 for hatching success was 30.5 ± 1.4 μg Ag L-1 and 6.04 ± 0.4 mg L-1 for AgNO3 and Ag ENMs, respectively. Further sub-lethal exposures were performed with the estimated LC10 concentrations for both AgNO3 or Ag ENMs over 96 h, where about 3.7% of the total Ag as AgNO3 was internalised, as measured by Ag accumulation in the dechorionated embryos. However, for the ENM exposures, nearly all (99.8%) of the total Ag was associated with chorion; indicating the chorion as an effective barrier to protect the embryo in the short term. Calcium (Ca2+) and sodium (Na+) depletion was induced in embryos by both forms of Ag, but hyponatremia was more pronounced in the nano form. Total glutathione (tGSH) levels declined in embryos exposed to both Ag forms, but a superior depletion occurred with the nano form. Nevertheless, oxidative stress was mild as superoxide dismutase (SOD) activity stayed uniform and the sodium pump (Na+/K+-ATPase) activity had no appreciable inhibition compared to the control. In conclusion, AgNO3 was more toxic to the early life stage zebrafish than the Ag ENMs, still differences were found in the exposure and toxic mechanisms of both Ag forms.

Wound infection is an important complicating factor in the wound healing process, and infections can be even more complex and difficult to manage in the case of wounds with biofilms. Silver has been used to treat infected wounds for a long time now, and the strength of the product depends on the number of Ag ions, where the greater the number of ions, the higher and faster the reactivity is. Ag Oxysalts technology-used in 3M Kerracontact Ag dressing-has three times more ions than standard silver dressings. The technology also does not show the typical disadvantages of silver, such as cytotoxicity and systemic toxicity. This article discusses the use of Ag Oxysalts technology for infected wounds and presents case studies to support the efficacy of this product in promoting wound healing.

Silver-based antimicrobials are widely used topically to treat infections associated with multi-drug resistant (MDR) pathogens. Expanding this topical use to aerosols to treat lung infections requires understanding and preventing silver toxicity in the respiratory tract. A key mechanism resulting in silver-induced toxicity is the production of reactive oxygen species (ROS). In this study, we have verified ROS generation in silver-treated bronchial epithelial cells prompting evaluation of three antioxidants, N-acetyl cysteine (NAC), ascorbic acid, and melatonin, to identify potential prophylactic agents. Among them, NAC was the only candidate that abrogated the ROS generation in response to silver acetate exposure resulting in the rescue of these cells from silver-associated toxicity. Further, this protective effect directly translated to preservation of metabolic activity, as demonstrated by the normal levels of citric acid cycle metabolites in NAC-pretreated silver acetate-exposed cells. Because the citric acid cycle remained functional, silver-exposed cells pre-incubated with NAC demonstrated significantly higher levels of adenosine triphosphate levels compared with NAC-free controls. Moreover, we found that this prodigious capacity of NAC to rescue silver acetate-exposed cells was due not only to its antioxidant activity, but also to its ability to directly bind silver. Despite binding to silver, NAC did not alter the antimicrobial activity of silver acetate.

Silver compounds have been used extensively for wound healing because of their antimicrobial properties, but high concentrations of silver are toxic to mammalian cells. We designed a peptide that binds silver and releases only small amounts of this ion over time, therefore overcoming the problem of silver toxicity. Silver binding was achieved through incorporation of an unnatural amino acid, 3\'-pyridyl alanine (3\'-PyA), into the peptide sequence. Upon the addition of silver ions, the peptide adopts a beta-sheet secondary structure and self-assembles into a strong hydrogel as characterized by rheology, circular dichroism, and transmission electron microscopy. We show that the resulting hydrogel kills Escherichia coli and Staphylococcus aureus but is not toxic to fibroblasts and could be used for wound healing. The amount of Ag(I) released by hydrogels into the solution is less than 4% and this low amount of Ag(I) does not change in the pH range 6-8. These studies provide an initial indication for use of the designed hydrogel as injectable, antimicrobial wound dressing.

Silver sulfadiazine (SSD) has been widely used in burned patients for the prevention of local infections. To be biologically active and exert antimicrobial properties, silver needs to be present in the form of silver ions (Ag1+) that bind to negatively charged proteins, namely, the RNA and DNA in microorganisms. However, previous published studies conducted with SSD in the 1990s reported a high level of silver absorption through damaged skin and noted the potential cytotoxicity of Ag1+ to human cells. SSD toxicity, however, had been described in cell cultures using arbitrary silver concentrations. In the present study, we determined the serum silver levels in burned patients treated with SSD and, taking into account the molar Ag1+ concentrations found in these patients, we evaluated the Ag1+ toxicity effects on inflammatory cells (ROS and cytokine production) in vitro.
Twenty patients with an average burned body surface area of 27.68% were included in this study.
Patients\' Ag1+ serum levels reached up to 558 times those of the unexposed controls. Ag1+ was then added to inflammatory cells in vitro at levels up to 2000 times the level of the control, and there was no effect on the viability of the cells nor on the rate of apoptosis. We observed a decrease in reactive oxygen species production by mononuclear (MN) and polymorphonuclear (PMN) cells, as well as a substantial decrease in cytokines IL-1β, IL-6, IL-8, IL-10, and TNF-α production by leukocytes (MN and PNM).
These findings suggest that Ag1+ may contribute to negative outcomes after burns, decreasing the primary defense mechanism (respiratory burst) and altering cytokine production.

Silver nanoparticles (AgNPs) have well-known antibacterial properties that have stimulated their widespread production and usage, which nonetheless concomitantly raises concerns regarding their release into the environment. Understanding the toxicity of AgNPs to biological systems, the environment, and the role that each silver species (Ag+ ions vs AgNPs) plays in that toxicity has received significant attention. One of the critical objectives of this research is the development of a reliable method that can sense and differentiate free silver ions from AgNPs and is able to characterize silver ions leaching from nanosilver. A number of analytical methods described in the literature that are available for sensing silver ions are costly, time consuming, tedious, and, more importantly, destroy the AgNP sample. To address these issues, a phosphorescent gold(I)-pyrazolate cyclic trinuclear complex (AuT) known to detect free silver ions was employed to detect and differentiate silver ions from AgNPs within an AgNP sample. The advantage of the proposed silver sensor is its ratiometric emission capability that undermines any background interference. The sensor exhibits a strong red emission (λmax = ∼690 nm) that, in the presence of Ag+ ions, will form a bright-green emissive adduct with a blue-shifted peak maximum near 475 nm yet red-shifted excitation peak. The presence of AgNPs did not inhibit the silver detection and quantification ability of the phosphorescent silver sensor. To understand the chemical transformation of nanosilver, the leaching of silver ions from AgNPs over a period of 35 days was monitored and quantified by measuring the I/ Io changes of the sensor. Furthermore, through adduct formation, the AuT molecular system was able to remediate free silver ions from the solution. The stronger affinity of the AuT complex to \"sandwich\" free silver ions than AgNPs was demonstrated in the presence of KCl salt that is well documented to form AgCl in the presence of silver ions. To our knowledge, this is the only ratiometric luminescence-based silver sensor able to successfully differentiate between Ag+ ions and AgNPs, sense the silver leakage from AgNPs, and remediate toxic silver ions from an aqueous solution. The synthesis and characterization of this sensor is a simple, single-step process-anticipating its viability for various applications.

Due to their strong antimicrobial activity, silver nanoparticles (AgNPs) are massively produced, applied, consumed and, as a negative consequence, released into wastewater treatment plants. Most AgNPs are assumed to be bound by sludge, and thus bear potential risk for microbial performance and stability. In this lab-scale study, flow cytometry as a high-throughput method and 16S rRNA gene amplicon Illumina MiSeq sequencing were used to track microbial community structure changes when being exposed to AgNPs. Both methods allowed deeper investigation of the toxic impact of chemicals on microbial communities than classical EC50 determination. In addition, ecological metrics were used to quantify microbial community variations depending on AgNP types (10 and 30 nm) and concentrations. Only low changes in α- and intra-community β-diversity values were found both in successive negative and positive control batches and batches that were run with AgNPs below the EC50 value. Instead, AgNPs at EC50 concentrations caused upcoming of certain and disappearance of formerly dominant subcommunities. Flavobacteriia were among those that almost disappeared, while phylotypes affiliated with Gammaproteobacteria (3.6-fold) and Bacilli (8.4-fold) increased in cell abundance in comparison to the negative control. Thus, silver amounts at the EC50 value affected community structure suggesting a potential negative impact on functions in wastewater treatment systems.

Silver nanoparticles (AgNPs) have potential uses in many applications, but current chemical production methods are challenged by scalability, limited particle stability, and the use of hazardous chemicals. The biological processes present in bacteria to mitigate metallic contaminants in their environment present a potential solution to these challenges. Before commercial exploitation of this technology can be achieved, the quality of bacteriogenic AgNPs needs to be improved for certain applications. While the colloidal and morphological stabilities of biogenic AgNPs are widely regarded as superior to chemogenic particles, little control over the synthesis of particle morphologies has been achieved in biological systems. This article reviews a range of biosynthetic reaction conditions and how they affect AgNP formation in bacteria to understand which are most influential. While there remains uncertainty, some general trends are emerging: higher Ag+ concentrations result in higher AgNP production, up to a point at which the toxic effects begin to dominate; the optimal temperature appears to be heavily species-dependent and linked to the optimal growth temperature of the organism. However, hotter conditions generally favor higher production rates, while colder environments typically give greater shape diversity. Little attention has been paid to other potentially important growth conditions including halide concentrations, oxygen exposure, and irradiation with light. To fully exploit biosynthetic production routes as alternatives to chemical methods, hurdles remain with controlling particle morphologies and require further work to elucidate and harness them. By better understanding the factors influencing AgNP production, a foundation can be laid from which shape-controlled production can be achieved.

It is essential to understand the mechanisms by which a toxicant is capable of poisoning the bacterial cell. The mechanism of action of many biocides and toxins, including numerous ubiquitous compounds, is not fully understood. For example, despite the widespread clinical and commercial use of silver (Ag), the mechanisms describing how this metal poisons bacterial cells remains incomplete. To advance our understanding surrounding the antimicrobial action of Ag, we performed a chemical genetic screen of a mutant library of Escherichia coli—the Keio collection, in order to identify Ag sensitive or resistant deletion strains. Indeed, our findings corroborate many previously established mechanisms that describe the antibacterial effects of Ag, such as the disruption of iron-sulfur clusters containing proteins and certain cellular redox enzymes. However, the data presented here demonstrates that the activity of Ag within the bacterial cell is more extensive, encompassing genes involved in cell wall maintenance, quinone metabolism and sulfur assimilation. Altogether, this study provides further insight into the antimicrobial mechanism of Ag and the physiological adaption of E. coli to this metal.