Effects of broth composition and light condition on antimicrobial susceptibility testing of ionic silver

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.

High value valorization of lignin as environmental benign antimicrobial

Lignin is a natural aromatic polymer of p-hydroxyphenylpropanoids with various biological activities. Noticeably, plants have made use of lignin as biocides to defend themselves from pathogen microbial invasions. Thus, the use of isolated lignin as environmentally benign antimicrobial is believed to be a promising high value approach for lignin valorization. On the other hand, as green and sustainable product of plant photosynthesis, lignin should be beneficial to reduce the carbon footprint of antimicrobial industry. There have been many reports that make use of lignin to prepare antimicrobials for different applications. However, lignin is highly heterogeneous polymers different in their monomers, linkages, molecular weight, and functional groups. The structure and property relationship, and the mechanism of action of lignin as antimicrobial remains ambiguous. To show light on these issues, we reviewed the publications on lignin chemistry, antimicrobial activity of lignin models and isolated lignin and associated mechanism of actions, approaches in synthesis of lignin with improved antimicrobial activity, and the applications of lignin as antimicrobial in different fields. Hopefully, this review will help and inspire researchers in the preparation of lignin antimicrobial for their applications.

Lignin from oil palm empty fruit bunches: Characterization, biological activities and application in green synthesis of silver nanoparticles

Lignin was extracted from oil palm empty fruit bunches under four different conditions. The lignin samples were characterized and employed in the green synthesis of silver nanoparticles. Two-dimensional HSQC NMR analysis showed that lignins extracted under more aggressive conditions (3.5% acid, 60 min) exhibited less signals and thus, presented a more degraded chemical structure. Additionally, those lignins obtained under harsh conditions (3.5% acid, 60 min) exhibited higher antioxidant capacity than those obtained under mild conditions (1.5% acid, 20 min). Formation of lignin-mediated silver nanoparticles was confirmed by color change during their synthesis. The surface plasmon resonance peaks (423-427 nm) in UV-visible spectra also confirmed the synthesis of AgNPs. AgNPs showed spherical shape, polycrystalline nature and average size between 18 and 20 nm. AgNPs, in suspension, presented a negative Zeta potential profile. Lignin was assumed to contribute in the antioxidant capacity exhibited by AgNPs. All AgNPs presented no significant differences on the disk diffusion antimicrobial susceptibility test against E. coli. The minimum inhibitory concentration of HAL3-L AgNPs (62.5 μg·mL^-1) was better than other physicochemically produced AgNPs (100 μg·mL^-1).

Beyond the Nanomaterials Approach: Influence of Culture Conditions on the Stability and Antimicrobial Activity of Silver Nanoparticles

Silver nanoparticles (AgNPs) as antimicrobial agents have been extensively studied. It is generally assumed that their inhibitory activity heavily depends on their physicochemical features. Yet, other parameters may affect the AgNP traits and activity, such as culture medium composition, pH, and temperature, among others. In this work, we evaluated the effect of the culture medium physicochemical traits on both the stability and antibacterial activity of AgNPs. We found that culture media impact the physicochemical traits of AgNPs, such as hydrodynamic size, surface charge, aggregation, and the availability of ionic silver release rate. As a consequence, culture media play a major role in AgNP stability and antimicrobial potency. The AgNP minimal inhibitory concentration (MIC) values changed up to 2 orders of magnitude by the influence of culture media alone when single-stock AgNPs were tested on the same strain of Escherichia coli. Furthermore, a meta-analysis of the AgNP MIC values confirms that the "chemical complexity" of culture media influences the AgNP activity. Studies that address only the antimicrobial activities of nanoparticles on common bacterial models should be performed by standardized susceptibility assays, thus generating replicable, comparable reports regarding the antimicrobial potency of nanomaterials.

Moderate temperature elevation increase susceptibility of early-life stage of the Mediterranean mussel, Mytilus galloprovincialis to metal-induced genotoxicity

The present study aims to evaluate the effects of copper and silver alone or along with a moderate temperature increase on embryonic development, DNA integrity and target gene expression levels in early life stages of Mytilus galloprovincialis. For this purpose, upon fertilized embryos were exposed to a sub-lethal concentration of Cu (9.54 μg/L), Ag (2.55 μg/L) and to the mixture of the two metals (Cu (6.67 μg/L) + Ag (1.47 μg/L)) along with a temperature gradient (18, 20 and 22 °C). In all experiments, larvae were exposed to stressors for 48 h except for those designed to DNA damage analysis exposed only for 24 h (before shell formation).Our results showed a significant increase in the percentage of malformed D-larvae (p < 0.05) with increasing temperature and exposure to silver and copper alone or in a mixture. Moreover, metal toxicity increased significantly (p < 0.05) with the temperature rise. Genotoxicity was evaluated using classic and modified with Formamidopyrimidine DNA glycosylase (Fpg) Comet assay. Results suggest that co-exposure to metals and temperature significantly increased DNA damage on mussel larvae with a more accentuated oxidative damage. A significant transcription modulation was observed for genes involved in DNA repair and DNA replication (p53, DNA ligase II and topoisomerase II) when larvae are exposed to a single stressor. However, in the case of multiple stresses, caspase involved in the cell apoptosis pathway was overexpressed. Our study suggests that mussel larvae exposed to a moderate increase in temperature may have a compromised ability to defend against genotoxicity. This is particularly relevant in the context of global warming and thermal pollution.

Seasonal variations of heavy metals content in mussels (Mytilus galloprovincialis) from Cala Iris offshore (Northern Morocco)

Heavy metal concentrations of Cd, Cr, Cu, Fe, Ni, Zn, Co, and Pb were investigated in soft tissues of Mytilus galloprovincialis coming from an aquaculture farm in Cala Iris sea of AlHoceima. Mytilus galloprovincialis were collected monthly during the period January to December 2016. The seasonal variations were affected significantly the concentrations of metals (Cd, Fe and Cr) in M. galloprovincialis. The highest heavy metal concentrations were recorded in winter (0.89 mg/kg, 673.2 and 3.330 mg/kg; for Cd, Fe and Cr, respectively) and the lowest values were founded in summer for Cd (0.646 mg/kg), and in autumn for Fe (340.1 mg/kg) and Cr (1.959 mg/kg). A significant effect of seasons on metal concentrations can be attributed to a number of biological and environmental inter-related factors. Data from this study may provide information on the use of M. galloprovincialis as a bioindicator for heavy metals pollution in the Cala Iris Sea.

Singlet oxygen formation in bio-inspired synthesis of a hollow Ag@AgBr photocatalyst for microbial and chemical decontamination

Singlet oxygen has been identified as a contributor to the degradation of contaminants using biosynthesised hollow Ag@AgBr catalysts.

High sensitivity of embryo-larval stage of the Mediterranean mussel, Mytilus galloprovincialis to metal pollution in combination with temperature increase

The present work aimed to assess the effects of two widespread metallic pollutants, copper and silver, along with environmentally-realistic temperature increases, on embryo-larval development of the Mediterranean mussel Mytilus galloprovincialis. First, mussel embryos upon fertilization were exposed for 48 h to increasing concentrations of Cu (0.5-500 μg/L) and Ag (0.1-100 μg/L) at different temperatures (18, 20, 22 or 24 °C) in order to characterize toxicity of each toxicant at the different tested temperatures. Increasing concentrations of a Cu-Ag mixture were then tested in order to assess the mixture effect at different temperatures (18, 20 or 22 °C). Embryotoxicity was measured after 48 h of exposure (D-larvae stage) considering both the percentage of abnormalities and developmental arrest in D-larvae. The results suggest that the optimum temperature for mussel larvae development is 18 °C (12.65± 1.6% malformations) and beyond 20 °C a steep increase of abnormal larvae was observed up to 100% at 24 °C. Ag was more toxic than Cu with a 50% effective concentration (EC50) at 18 °C of 6.58 μg/L and 17.6 μg/L, respectively. Temperature increased the toxicity of both metals as proved with the EC50 at 20 °C at 3.86 μg/L and 16.28 μg/L for Ag and Cu respectively. Toxic unit calculation suggests additive effects of Cu and Ag in mixture at 18 and 20 °C. These results highlight a possible impairment of M. galloprovincialis reproduction in the Mediterranean Sea in relation to increase of both pollutants and water temperature due to global warming.