New insights of the bacterial response to exposure of differently sized silver nanomaterials

Ammonia monooxygenase-mediated cometabolic biotransformation of volatile 4-chlorophenol in nitrifying counter-diffused biofilms: A combined molecular dynamics simulation, DFT calculation and experimental study

Ammonia monooxygenase (AMO)-mediated cometabolism of organic pollutants has been widely observed in biological nitrogen removal process. However, its molecular mechanism remains unclear, hindering its practical application. Furthermore, conventional nitrification systems encounter significant challenges such as air pollution and the loss of ammonia-oxidizing bacteria, when dealing with wastewater containing volatile organic pollutants. This study developed a nitrifying membrane-aerated biofilm reactor (MABR) to enhance the biodegradation of volatile 4-chlorophenol (4-CP). Results showed that 4-CP was primarily removed via Nitrosomonas nitrosa-mediated cometabolism in the presence of NH4+-N, supported by the increased nicotinamide adenine dinucleotide (NADH) and adenosine triphosphate (ATP) content, AMO activity and the related genes abundance. Hydroquinone, detected for the first time and produced via oxidative dechlorination, as well as 4-chlorocatechol was primary transformation products of 4-CP. Nitrosomonas nitrosa AMO structural model was constructed for the first time using homology modeling. Molecular dynamics simulation suggested that the ortho-carbon in the benzene ring of 4-CP was more prone to metabolismcompared to the ipso-carbon. Density functional theory calculation revealed that 4-CP was metabolized by AMO via H-abstraction-OH-rebound reaction, with a significantly higher rebound barrier at the ipso-carbon (16.37 kcal·mol-1) as compared to the ortho-carbon (6.7 kcal·mol-1). This study fills the knowledge gap on the molecular mechanism of AMO-mediated cometabolism of organic pollutants, providing practical and theoretical foundations for improving volatile organic pollutants removal through nitrifying MABR.

Mechanism Involved in Polyvinyl Chloride Nanoplastics Induced Anaerobic Granular Sludge Disintegration: Microbial Interaction Energy, EPS Molecular Structure, and Metabolism Functions

Nanoplastics (NPs) are emerging pollutants and have been reported to cause the disintegration of anaerobic granular sludge (AnGS). However, the mechanism involved in AnGS disintegration was not clear. In this study, polyvinyl chloride nanoplastics (PVC-NPs) were chosen as target NPs and their long-term impact on AnGS structure was investigated. Results showed that increasing PVC-NPs concentration resulted in the inhibition of acetoclastic methanogens, syntrophic propionate, and butyrate degradation, as well as AnGS disintegration. At the presence of 50 μg·L-1 PVC-NPs, the hydrophobic interaction was weakened with a higher energy barrier due to the relatively higher hydrophilic functional groups in extracellular polymeric substances (EPS). PVC-NPs-induced ROS inhibited quorum sensing, significantly downregulated hydrophobic amino acid synthesis, whereas it highly upregulated the genes related to the synthesis of four hydrophilic amino acids (Cys, Glu, Gly, and Lys), resulting in a higher hydrophily degree of protein secondary structure in EPS. The differential expression of genes involved in EPS biosynthesis and the resulting protein secondary structure contributed to the greater hydrophilic interaction, reducing microbial aggregation ability. The findings provided new insight into the long-term impact of PVC-NPs on AnGS when treating wastewater containing NPs and filled the knowledge gap on the mechanism involved in AnGS disintegration by PVC-NPs.

Nanomaterials Regulate Bacterial Quorum Sensing: Applications, Mechanisms, and Optimization Strategies

Anti-virulence therapy that interferes with bacterial communication, known as "quorum sensing (QS)", is a promising strategy for circumventing bacterial resistance. Using nanomaterials to regulate bacterial QS in anti-virulence therapy has attracted much attention, which is mainly attributed to unique physicochemical properties and excellent designability of nanomaterials. However, bacterial QS is a dynamic and multistep process, and there are significant differences in the specific regulatory mechanisms and related influencing factors of nanomaterials in different steps of the QS process. An in-depth understanding of the specific regulatory mechanisms and related influencing factors of nanomaterials in each step can significantly optimize QS regulatory activity and enhance the development of novel nanomaterials with better comprehensive performance. Therefore, this review focuses on the mechanisms by which nanomaterials regulate bacterial QS in the signal supply (including signal synthesis, secretion, and accumulation) and signal transduction cascade (including signal perception and response) processes. Moreover, based on the two key influencing factors (i.e., the nanomaterial itself and the environment), optimization strategies to enhance the QS regulatory activity are comprehensively summarized. Collectively, applying nanomaterials to regulate bacterial QS is a promising strategy for anti-virulence therapy. This review provides reference and inspiration for further research on the anti-virulence application of nanomaterials.

A systematic review of antibiotic resistance driven by metal-based nanoparticles: Mechanisms and a call for risk mitigation

Elevations in antibiotic resistance genes (ARGs) are due not only to the antibiotic burden, but also to numerous environmental pressures (e.g., pesticides, metal ions, or psychotropic pharmaceuticals), which have led to an international public health emergency. Metal-based nanoparticles (MNPs) poison bacteria while propelling nanoresistance at ambient or sub-lethal concentrations, acting as a wide spectrum germicidal agent. Awareness of MNPs driven antibiotic resistance has created a surge of investigation into the molecule mechanisms of evolving and spreading environmental antibiotic resistome. Co-occurrence of MNPs resistance and antibiotic resistance emerge in environmental pathogens and benign microbes may entail a crucial outcome for human health. In this review we expound on the systematic mechanism of ARGs proliferation under the stress of MNPs, including reactive oxygen species (ROS) induced mutation, horizontal gene transfer (HGT) relevant genes regulation, nano-property, quorum sensing, and biofilm formation and highlighting on the momentous contribution of nanoparticle released ion. As antibiotic resistance pattern alteration is closely knit with the mediate activation of nanoparticle in water, soil, manure, or sludge habitats, we have proposed a virulence and evolution based antibiotic resistance risk assessment strategy for MNP contaminated areas and discussed practicable approaches that call for risk management in critical environmental compartments.

Comprehensive response of microbes to Ag and Ag2S nanoparticles and silver spatial distribution in constructed wetlands

This study investigated functional bacteria, key enzymes, and nitrogen metabolism in vertical flow constructed wetlands (CWs) after exposing to silver, silver sulfide nanoparticles (Ag NPs and Ag2S NPs), and silver iron (Ag+), and silver spatial distribution in CWs for 155 days. Ag NPs and Ag2S NPs affected species richness and diversity whereas Ag+ showed the higher the species diversity indices. Sequencing analysis exhibited that Ag NPs or Ag+ significantly inhibited nitrogen metabolic process by hindering the relative activity of functional enzymes, downregulating relative abundances of nrfA, norB and napA for Ag NPs, nxrA gene for Ag+, while Ag2S NPs inhibited relative abundance of nirA. The above results confirmed that NPs or Ag+ significantly reduced nitrogen removal and Ag NPs mainly inhibited NO3--N removal while Ag+ significantly suppressed NH4+-N removal. This study also found that CWs could effectively remove NPs or Ag+ (about 98 %), and nanoparticles showed higher translocation factors (TFs) values (0.81-1.15 or 0.36), indicating nanoparticles transported easily through substrate layers.

Mechanism of escape from the antibacterial activity of metal-based nanoparticles in clinically relevant bacteria: A systematic review

The emergency of antibiotic-resistant bacteria in severe infections is increasing, especially in nosocomial environments. The ESKAPE group is of special importance in the groups of multi-resistant bacteria due to its high capacity to generate resistance to antibiotics and bactericides. Therefore, metal-based nanomaterials are an attractive alternative to combat them because they have been demonstrated to damage biomolecules in the bacterial cells. However, there is a concern about bacteria developing resistance to NPs and their harmful effects due to environmental accumulation. Therefore, this systematic review aims to report the clinically relevant bacteria that have developed resistance to the NPs. According to the results of this systematic review, various mechanisms to counteract the antimicrobial activity of various NP types have been proposed. These mechanisms can be grouped into the following categories: production of extracellular compounds, metal efflux pumps, ROS response, genetic changes, DNA repair, adaptative morphogenesis, and changes in the plasma membrane.

Marine toxin domoic acid alters nitrogen cycling in sediments

As a red tide algal toxin with intense neurotoxicity distributed worldwide, domoic acid (DA) has attracted increasing concerns. In this work, the integrative analysis of metagenome and metabolome are applied to investigate the impact of DA on nitrogen cycling in coastal sediments. Here we show that DA can act as a stressor to induce the variation of nitrogen (N) cycling by altering the abundance of functional genes and electron supply. Moreover, microecology theory revealed that DA can increase the role of deterministic assembly in microbial dynamic succession, resulting in the shift of niches and, ultimately, the alteration in N cycling. Notably, denitrification and Anammox, the important process for sediment N removal, are markedly limited by DA. Also, variation of N cycling implies the modification in cycles of other associated elements. Overall, DA is capable of ecosystem-level effects, which require further evaluation of its potential cascading effects.

Metatranscriptome deciphers the effects of non-antibiotic antimicrobial agents on antibiotic resistance and virulence factors in freshwater microcosms

The emergence and transmission of antibiotic resistance genes (ARGs) and virulence factors (VFs) pose health risks to the ecosystem and humans. Understanding how non-antibiotic antimicrobial agents drive the expression of ARGs and VFs in freshwater ecosystems, however, remains large challenges. Here, we employed freshwater microcosms and performed metatranscriptomic analysis to investigate the expression profiles of ARGs and VFs in response to pollutants of non-antibiotic antimicrobial agents, including silver nanoparticles (AgNPs) and azoxystrobin. Results showed that AgNPs significantly inhibited the total expression of ARGs and VFs and decreased the number of pathogenic microorganisms expressing these genes. Azoxystrobin increased the total expression of ARGs and VFs, as well as the number of pathogens expressing VFs, but concomitantly reduced the number of pathogens expressing ARGs. Two tested pollutants dramatically changed the expression profiles of ARGs and VFs, with distinct patterns: AgNPs displayed a negative effect, while azoxystrobin showed a positive effect on their expression profiles. Our findings provided a systematical insight to demonstrate that non-antibiotic antimicrobial agents with different mechanisms of action showed various effects on ARGs and VFs, and therefore represented different ecological risks.

Microplastics as potential carriers of viruses could prolong virus survival and infectivity

Microplastics are emerging contaminants in various aquatic environments, leading to human and environmental health concerns. Viruses have also been ubiquitously detected in aquatic environments, and there is an unknown risk of microplastics-mediated virus migration through adsorption. This study applied polystyrene microplastics as the carrier and the T4 bacteriophage (or phage) as the virus model, and a violet side scatter/green fluorescence double-gated flow cytometry approach to investigate the adsorption capacity of viruses on microplastics. Our results show that up to 98.6±0.2% of the dosed viruses can be adsorbed by microplastics, and such adsorptions are dependent on size and surface functional groups. Both Fourier-transform infrared spectroscopy and fluorescence-labelled confocal microscopy confirmed that the virus can successfully adsorb onto microplastics. Zeta potential characterisation revealed that the electrostatic interaction is the primary adsorption mechanism associated with the adsorption of viruses. UV-aging was found to enhance the adsorption capacities of viruses on microplastics. Both pristine and UV-aged microplastics were found to significantly prolong the infectivity of the adsorbed viruses, even under elevated temperatures. Collectively, our findings highlight that microplastics are associated with the biological risks of water-borne viral transmission through virus adsorption.

Pluripotency of endogenous AHL-mediated quorum sensing in adaptation and recovery of biological nitrogen removal system under ZnO nanoparticle long-term exposure

The impacts of quorum sensing (QS) on nanoparticle (NP)-stressed biological nitrogen removal (BNR) system have seldom been addressed yet. In this study, the contributions of endogenous N-acyl-homoserine lactone (AHL)-based QS regulation to the BNR system's adaptation to the zinc oxide (ZnO) NP stress and its recovery potential were systematically investigated. Although 1 mg/L ZnO NPs exerted little impact on the BNR system, chronic exposure to 10 mg/L ones depressed the system's BNR performance which irreversibly impaired the nitrification process even when the system entered the recovery period with no NP added anymore. Meanwhile, ZnO NPs exhibited hormesis effects on the production of AHLs and extracellular polymeric substance (EPS), and activities of superoxide dismutase and catalase. During the ZnO NP exposure period, C₄-HSL, C₆-HSL, and C₁₀-HSL were discovered to be positively associated with nitrogen removal efficiency, tightly-bound EPS production, and antioxidase activities. Besides, the shifts of Nitrospira, Dechloromonas, Aeromonas, Acinetobacter, Delftia, and Bosea were expected to determine the AHL's dynamic distribution. During the system's recovery stage, Dechloromonas replaced Candidatus_Competibacter as the dominant denitrification-related genus. Dechloromonas abundance elevated with the increased contents of C₄-HSL in the aqueous and EPS phases and C₁₀-HSL in EPS and sludge phases, and were expected to promote the activities of BNR-related and antioxidant enzymes, and the EPS production to assist in the recovery of the impaired system's BNR performance. The QS-related BNR genera exhibited higher resilience to ZnO NPs than quorum quenching-related ones, indicating their critical role in nitrogen removal in the restored system. This work provided an insight into the potential pluripotency of AHL-based QS regulation on the ZnO NP-stressed BNR system's adaptation and recovery.

Reactive nitrogen species from free nitrous acid (FNA) cause cell lysis

Free nitrous acid (FNA, i.e. HNO₂) has been demonstrated to have broad biocidal effects on a range of microorganisms, which has direct implications for wastewater management. However, the biocidal mechanisms still remain largely unknown. This study aims to test the hypothesis that FNA will induce cell lysis via cell membrane perforations, and consequently cause cell death via proteolysis, through the use of two model organisms namely Escherichia coli K12 and Pseudomonas putida KT2440. A combination of analytical techniques that included viability assays, atomic force microscopy (AFM), protein abundance assays and proteomic analysis using Quadruple-Orbitrap™ Mass spectrometry was used to evaluate the extent of cell death and possible cell lysis mechanisms. FNA treatment at 6.09 mg/L for 24 h (conditions typically applied in applications) induced 36 ± 4.2% and 91 ± 3.5% cell death/lysis of E. coli and P. putida, respectively. AFM showed that the lysis of cells was observed via perforations in the cell membrane; cells also appeared to shrink and become flat following FNA treatment. By introducing a reactive nitrogen species (RNS) scavenger to act as a treatment control, we further revealed that it was the nitrosative decomposition species of FNA, such as ^.NO that caused the cell lysis through the destruction of protein macromolecules found in the cell membrane (proteolysis). Subsequently, the RNS went on to cause the destruction of protein macromolecules within the cells. The death of these model organisms E. coli and P. putida following exposure to FNA treatment provides insights into the use of FNA as an antimicrobial agent in wastewater treatment.

Unveiling changes of microbial community involved in N and P removal in constructed wetlands with exposing to silver nanoparticles

Constructed wetlands (CWs) are environmentally friendly engineered systems to purify wastewater, with low-cost and easy maintenance. However, it is not clear on responses of functional microbes for nitrogen (N) and phosphorus (P) biotransformation in CWs to silver nanoparticles (Ag NPs). The high throughput sequencings were employed to reveal microbial communities in vertical flow subsurface CWs with stable operation for 120 days. The results indicated that NH₄⁺-N, TN and TP removal of soil layer decreased by 43.56%, 15.7% and 22.7% under stress of Ag NPs. Microbial richness index and compositions were affected, and control wetland enriched Sulfurospirillum, Desulfarculaceae and Flavobacterium whereas CWs exposed to Ag NPs enriched Desulfosporosinus and Desulfurispora from LEfSe analysis. Moreover, after dosing Ag NPs, relative abundances of functional genes amoA and hao for nitrification, nirK and norB for denitrification and ppx and phoA/phoD for phosphorus conversions in upper soil were significantly downregulated. Inhibition on functional bacteria and genes of Ag NPs explained poor removal efficiencies of nitrogen and phosphorus pollutants in CWs. Our findings give an insight into ecological toxicity of Ag NPs on CWs with N and P bioconversions and provide the understanding of response of nitrifiers, denitrifies and PAOs.

Self-sustained recovery of silver with stainless-steel based Cobalt/Molybdenum/Manganese polycrystalline catalytic electrode in bio-electroreduction microbial fuel cell (BEMFC)

In this study, a novel bio-electroreduction microbial fuel cell (BEMFC) assisted by stainless-steel based Cobalt/Molybdenum/Manganese (Co/Mo/Mn-SS) polycrystalline catalytic electrode was used to achieve high recovery to silver. The exoelectrogens (Shewanella sp. etc.) using organic wastewater (the inflow was controlled at 1.2 L d^-1) as nutrient matrix in the anode chamber generated electrons, while silver ions were simultaneously electroreduced and electrodeposited on the surface of the catalytic electrode as electron acceptors. Silver nanoplates could be observed directly. The products of electroreduction on the cathode were analyzed by Scanning Electron Microscopy (SEM), Transmission Electron Microscope (TEM), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffractometer (XRD), and the results of electrochemical characterization confirmed the existence of silver in the products. In the operation, the silver ions were in-situ recovered and enriched from the initial concentration of 20-300 mg L^-1 to almost complete recovery (8-18 h), with the maximum power density of 1008.2 mW m^-2 and 5.5 A m^-2 current density. The recovery efficiency of silver in the BEMFC using the Co/Mo/Mn-SS electrode was up to 9.60 kg m^-2h^-1, and the energy efficiency was 27.8 kg kWh^-1. Under the continuous flow operation mode, the BEMFC still achieved 90.2% recovery efficiency of the silver.

Advances in interfacial engineering for enhanced microbial extracellular electron transfer

The extracellular electron transfer (EET) efficiency between electroactive microbes (EAMs) and electrode is a key factor determining the development of microbial electrochemical technology (MET). Currently, the low EET efficiency of EAMs limits the application of MET in the fields of organic matter degradation, electric energy production, seawater desalination, bioremediation and biosensing. Enhancement of the interaction between EAMs and electrode by interfacial engineering methods brings bright prospects for the improvement of the EET efficiency of EAMs. In view of the research in recent years, this mini-review systematically summarizes various interfacial engineering strategies ranging from electrode surface modification to hybrid biofilm formation, then to single cell interfacial engineering and intracellular reformation for promoting the electron transfer between EAMs and electrode, focusing on the applicability and limitations of these methodologies. Finally, the possible key directions, challenges and opportunities for future interfacial engineering to strengthen the microbial EET are proposed in this mini-review.

Perturbation of clopyralid on bio-denitrification and nitrite accumulation: Long-term performance and biological mechanism

The contaminant of herbicide clopyralid (3,6-dichloro-2- pyridine-carboxylic acid, CLP) poses a potential threat to the ecological system. However, there is a general lack of research devoted to the perturbation of CLP to the bio-denitrification process, and its biological response mechanism remains unclear. Herein, long-term exposure to CLP was systematically investigated to explore its influences on denitrification performance and dynamic microbial responses. Results showed that low-concentration of CLP (<15 mg/L) caused severe nitrite accumulation initially, while higher concentrations (35-60 mg/L) of CLP had no further effect after long-term acclimation. The mechanistic study demonstrated that CLP reduced nitrite reductase (NIR) activity and inhibited metabolic activity (carbon metabolism and nitrogen metabolism) by causing oxidative stress and membrane damage, resulting in nitrite accumulation. However, after more than 80 days of acclimation, almost no nitrite accumulation was found at 60 mg/L CLP. It was proposed that the secretion of extracellular polymeric substances (EPS) increased from 75.03 mg/g VSS at 15 mg/L CLP to 109.97 mg/g VSS at 60 mg/L CLP, which strengthened the protection of microbial cells and improved NIR activity and metabolic activities. Additionally, the biodiversity and richness of the microbial community experienced a U-shaped process. The relative abundance of denitrification- and carbon metabolism-associated microorganisms decreased initially and then recovered with the enrichment of microorganisms related to the secretion of EPS and N-acyl-homoserine lactones (AHLs). These microorganisms protected microbe from toxic substances and regulated their interactions among inter- and intra-species. This study revealed the biological response mechanism of denitrification after successive exposure to CLP and provided proper guidance for analyzing and treating herbicide-containing wastewater.

Experimental evolution of Pseudomonas putida under silver ion versus nanoparticle stress

Whether the antibacterial properties of silver nanoparticles (AgNPs) are simply due to the release of silver ions (Ag⁺ ) or, additionally, nanoparticle-specific effects, is not clear. We used experimental evolution of the model environmental bacterium Pseudomonas putida to ask whether bacteria respond differently to Ag⁺ or AgNP treatment. We pre-evolved five cultures of strain KT2440 for 70 days without Ag to reduce confounding adaptations before dividing the fittest pre-evolved culture into five cultures each, evolving in the presence of low concentrations of Ag⁺ , well-defined AgNPs or Ag-free controls for a further 75 days. The mutations in the Ag⁺ or AgNP evolved populations displayed different patterns that were statistically significant. The non-synonymous mutations in AgNP-treated populations were mostly associated with cell surface proteins, including cytoskeletal membrane protein (FtsZ), membrane sensor and regulator (EnvZ and GacS) and periplasmic protein (PP_2758). In contrast, Ag⁺ treatment was selected for mutations linked to cytoplasmic proteins, including metal ion transporter (TauB) and those with metal-binding domains (ThiL and PP_2397). These results suggest the existence of AgNP-specific effects, either caused by sustained delivery of Ag⁺ from AgNP dissolution, more proximate delivery from cell-surface bound AgNPs, or by direct AgNP action on the cell's outer membrane.

Transcriptomics and Metabolomics Revealed the Biological Response of Chlorella pyrenoidesa to Single and Repeated Exposures of AgNPs at Different Concentrations

Increased release of engineered nanoparticles (ENPs) from widely used commercial products has threatened environmental health and safety, particularly the repeated exposures to ENPs with relatively low concentration. Herein, we studied the response of Chlorella pyrenoidesa (C. pyrenoidesa) to single and repeated exposures to silver nanoparticles (AgNPs). Repeated exposures to AgNPs promoted chlorophyll a and carotenoid production, and increased silver accumulation, thus enhancing the risk of AgNPs entering the food chain. Notably, the extracellular polymeric substances (EPS) content of the 1-AgNPs and 3-AgNPs groups were dramatically increased by 119.1% and 151.5%, respectively. We found that C. pyrenoidesa cells exposed to AgNPs had several significant alterations in metabolic process and cellular transcription. Most of the genes and metabolites are altered in a dose-dependent manner. Compared with the control group, single exposure had more differential genes and metabolites than repeated exposures. 562, 1341, 4014, 227, 483, and 2409 unigenes were differentially expressed by 1-0.5-AgNPs, 1-5-AgNPs, 1-10-AgNPs, 3-0.5-AgNPs, 3-5-AgNPs, and 3-10-AgNPs treatment groups compared with the control. Metabolomic analyses revealed that AgNPs altered the levels of sugars and amino acids, suggesting that AgNPs reprogrammed carbon/nitrogen metabolism. The changes of genes related to carbohydrate and amino acid metabolism, such as citrate synthase (CS), isocitrate dehydrogenase (IDH1), and malate dehydrogenase (MDH), further supported these results. These findings elucidated the mechanism of biological responses to repeated exposures to AgNPs, providing a new perspective on the risk assessment of nanomaterials.

Response of extracellular polymeric substances and microbial community structures on resistance genes expression in wastewater treatment containing copper oxide nanoparticles and humic acid

The extracellular polymeric substances (EPS) and microbial community structures were investigated in wastewater treatment containing copper oxide nanoparticles (CuO NPs) (reactor R1) and CuO NPs and humic acid (HA) (reactor R2) using both sequencing batch bioreactors (SBRs), and their response on resistance genes expression was analyzed. The removal of influent chemical oxygen demands (COD) and NH₄⁺-N was moderately influenced under CuO NPs (5 mg/L) stress, while the function of HA (10 mg/L) was not reflected. However, the EPS production and microbial community were affected by the HA addition. The expression of different antibiotic resistance genes (ARGs), metal-resistance genes (MRGs), and intI1 was related to the primary compositions of polysaccharides and proteins in EPS and different microbial communities at the genus level. Furthermore, the expression of resistance genes was not stimulated under CuO NPs stress, and supplying HA was suggested to reduce their expression in wastewater treatment.

Bioavailability and toxicity of silver nanoparticles: Determination based on toxicokinetic-toxicodynamic processes

Determining the bioavailability and toxicity mechanism of silver nanoparticles (AgNPs) is challenging as Ag⁺ is continuously released by external or internal AgNP dissolution in the actual exposure system (regardless of the laboratory or the natural environment). Here a novel pulsed-gradient Ag⁺ (AgNO₃) exposure was conducted with zebrafish (Danio rerio) larvae to simulate dissolved gradient concentrations of Ag⁺ from polyvinylpyrrolidone (PVP)-coated AgNPs. The accumulation and toxicity of the pulsed-gradient Ag⁺ (AgNO₃) and, in the meantime, the released Ag⁺ from PVP-AgNPs were predicted using a toxicokinetic-toxicodynamic (TK-TD) model with obtained Ag⁺ parameters. In order to further understand the possible mechanism of PVP-AgNP releasing Ag⁺ in the body, subcellular fractions (S9) of zebrafish were also used to incubate with AgNPs in vitro to mimic the realistic in vivo scenarios. In the TK process, in vivo analysis showed that AgNPs released around twice as many Ag⁺ into the body than were detected with a single Ag⁺ pulse-exposure system; this was supported by evidence that subcellular S9 fractions might cause the PVP-AgNPs to lose the capping agent and favor Ag⁺ release. In the TD process, toxicity (survival rate) was predicted by the total bodily Ag(I) concentration, suggesting that AgNP toxicity in larvae was mainly due to gradually released Ag⁺ rather than AgNPs themselves. This study helps clarify the role of Ag⁺ in AgNP toxicity and offers a novel framework by which to investigate the toxicity of metal nanoparticles and corresponding metal ions in biological systems.

Biogenic silver nanoparticle (Bio-AgNP) has an antibacterial effect against carbapenem-resistant Acinetobacter baumannii with synergism and additivity when combined with polymyxin B

AIMS

Carbapenem-resistant Acinetobacter baumannii represents a public health problem, and the search for new antibacterial drugs has become a priority. Here, we investigate the antibacterial activity of biogenic silver nanoparticles (Bio-AgNPs) synthesized by Fusarium oxysporum, used alone or in combination with polymyxin B against carbapenem-resistant A. baumannii.

METHODS AND RESULTS

In this study, ATCC^® 19606^™ strain and four carbapenem-resistant A. baumannii strains were used. The antibacterial activity of Bio-AgNPs and its synergism with polymyxin B were determined using broth microdilution, checkboard methods and time-kill assays. The integrity of the bacterial cell membrane was monitored by protein leakage assay. In addition, the cytotoxicity in the VERO mammalian cell line was also evaluated, and the selectivity index was calculated. Bio-AgNPs have an antibacterial activity with MIC and MBC ranging from 0.460 to 1.870 µg/ml. The combination of polymyxin B and Bio-AgNPs presents synergy against four of the five strains tested and additivity against one strain in the checkerboard assay. Considering the time of cell death, Bio-AgNPs killed all carbapenem-resistant isolates and ATCC^® 19606^™ within 1 h. When combined, Bio-AgNPs presented 16-fold reduction of the polymyxin B MIC and showed a decrease in terms of viable A. baumannii cells in 4 h of treatment, with synergic and additive effects. Protein leakage was observed with increasing concentrations for Bio-AgNPs treatments. Additionally, Bio-AgNP and polymyxin B showed dose-dependent cytotoxicity against mammalian VERO cells and combined the cytotoxicity which was significantly reduced and presented a greater pharmacological safety.

CONCLUSIONS

The results presented here indicate that Bio-AgNPs in combination with polymyxin B could represent a good alternative in the treatment of carbapenem-resistant A. baumannii.

SIGNIFICANCE AND IMPACT OF STUDY

This study demonstrates the synergic effect between Bio-AgNPs and polymyxin B on carbapenem-resistant A. baumannii strains.

Epigenetic effects of silver nanoparticles and ionic silver in Tetrahymena thermophila

The widespread use of silver nanoparticles (Ag NPs) has raised substantial health risks, but little is known about the epigenetic toxicity induced by Ag⁺ and Ag NPs. This study characterized physiological and lncRNA profiles to explore the toxic effects and epigenetic mechanisms in Tetrahymena thermophila on exposure to Ag⁺ (in the form of AgNO₃) and different Ag NPs for 24 h. The Ag NPs studied varied in size (10 nm and 80 nm) and surface coating (citrate and polyvinylpyrrolidone). We found that both Ag⁺ and Ag NPs elicited strong growth-inhibiting effects on T. thermophila. The toxicity was mainly caused by high reactive oxygen species (ROS) levels, leading to lipid peroxidation and mitochondrial dysfunction. To combat the oxidative stress, the protist activated an antioxidative response, increasing the activity of glutathione peroxidase and other antioxidants. Notably, 1250 lncRNAs were differentially expressed under Ag⁺ or Ag NPs exposure relative to the non-exposure control, which were clustered into 15 expression modules in weighted gene co-expression network analysis. These gene modules exhibited toxicant-specific expression patterns, potentially playing regulatory roles, via their co-expressed mRNAs, to inhibit cell growth, activate cell membrane cation channel, and promote oxidoreductase activity. This research illuminates how post-transcriptional mechanisms of a ciliated protozoan regulate responses to Ag⁺ and Ag NPs toxicities.

Antimicrobial and Antibiofilm Activities of New Synthesized Silver Ultra-NanoClusters (SUNCs) Against Helicobacter pylori

Helicobacter pylori colonizes approximately 50% of the world’s population, and it is the cause of chronic gastritis, peptic ulcer disease, and gastric cancer. The increase of antibiotic resistance is one of the biggest challenges of our century due to its constant increase. In order to identify an alternative or adjuvant strategy to the standard antibiotic therapy, the in vitro activity of newly synthesized Silver Ultra-NanoClusters (SUNCs), characterized by an average size inferior to 5 nm, against clinical strains of H. pylori, with different antibiotic susceptibilities, was evaluated in this study. MICs and MBCs were determined by the broth microdilution method, whereas the effect of drug combinations was determined by the checkerboard assay. The Minimum Biofilm Eradication Concentration (MBEC) was measured using AlamarBlue (AB) assay and colony-forming unit (CFU) counts. The cytotoxicity was evaluated by performing the MTT assay on the AGS cell line. The inhibitory activity was expressed in terms of bacteriostatic and bactericidal potential, with MIC₅₀, MIC₉₀, and MBC₅₀ of 0.33 mg/L against planktonic H. pylori strains. Using the fractional inhibitory concentration index (FICI), SUNCs showed potential synergism with metronidazole and clarithromycin. The biofilm eradication was obtained after treatment with 2×, 3×, and 4× MIC values. Moreover, SUNCs showed low toxicity on human cells and were effective in eradicating a mature biofilm produced by H. pylori. The data presented in this study demonstrate that SUNCs could represent a novel strategy for the treatment of H. pylori infections either alone or in combination with metronidazole.