The rise of multidrug-resistant bacteria (MDRB) poses a significant threat to global health. The search for alternative antimicrobial agents is crucial, and nanomaterials, particularly silver nanomaterials (AgNMs), have shown promise. While silver nanoparticles (AgNPs) have been studied extensively, their efficacy against MDRB needs improvement, and resistance development remains a concern. This study investigated the potential of silver nanoclusters (AgNCs) as a superior alternative. Previous research has highlighted several mechanisms through which nanomaterials combat bacterial resistance: membrane damage, reactive oxygen species (ROS) production, enzyme-mimicking activity, ion release (e.g., silver ions), and biofilm eradication. Silver nanomaterials, especially AgNPs, are among the most effective antimicrobial nanomedicines due to their wide range of applications. However, the antimicrobial activity of AgNCs remains largely unexplored. This research aimed to optimize the size, surface hydrophilicity/hydrophobicity, and core fractioning of AgNMs to enhance their antimicrobial properties. The study hypothesized that small, amphiphilic AgNCs would exhibit superior antimicrobial activity compared to conventional AgNPs against MDR Pseudomonas aeruginosa, both in vitro and in vivo.

The researchers conducted a comprehensive literature review using keywords such as "silver nanoparticles," "nanosilver," "Ag," "antibacterial," "antimicrobial," and "bacterial" in Web of Science and PubMed. They screened over 1700 publications, applying inclusion and exclusion criteria to select 266 datasets from 36 papers. The criteria focused on non-doped AgNCs with clearly measured sizes under 100 nm, demonstrated antibacterial effects via conventional methods (MIC, antibacterial efficiency), chemical synthesis with defined surface modifications, and clearly described bacterial strains. Excluded were studies on nonsensible homogenous systems (textiles, creams, etc.) and those using green synthesis methods with unclear surface modifications. A random forest model was used to assess the importance of various factors (size, surface ligand, surface charge, etc.) in determining minimum inhibitory concentration (MIC) values. This analysis revealed that size and surface ligands were the most influential factors. A general linear regression model was also employed to analyze the correlation between particle size and MIC values.

The study involved the synthesis and characterization of AgNCs and AgNPs. AgNCs were synthesized in two steps: first, a hydrophobic core was synthesized using a biphasic reaction, and then an amphiphilic shell was created through ligand exchange with mercaptosuccinic acid (MSA). AgNPs were synthesized using similar methods, with variations in MSA and 1-adamantanethiol (S-Adm) concentrations. The characterization of the nanoparticles included analyzing their size, morphology, and surface properties (hydrophilicity/hydrophobicity). The antibacterial activity of AgNCs and AgNPs was evaluated against various MDR bacterial strains, including Pseudomonas aeruginosa, Acinetobacter baumannii, and Escherichia coli. Minimum inhibitory concentration (MIC) values were determined. The mechanisms of action were explored through studies examining cell membrane damage, ROS production, and effects on cellular processes such as ATP synthesis. In vivo studies were conducted using a mouse model of Pseudomonas aeruginosa pneumonia to assess the therapeutic efficacy and biocompatibility of AgNCs.

The study found that AgNCs exhibited significantly higher antibacterial activity compared to AgNPs against MDR Pseudomonas aeruginosa. The amphiphilic nature of AgNCs facilitated their interaction with bacterial cell membranes and enhanced endocytosis. The nanocluster structure led to strong peroxide-like activity and significant ROS production, contributing to their potent bactericidal effect. AgNCs demonstrated efficacy against bacterial cell membranes, induced oxidative stress, and inhibited ATP synthesis. In the in vivo mouse pneumonia model, AgNCs significantly improved survival rates compared to the control group, showcasing excellent therapeutic potential while demonstrating good biocompatibility. The random forest model analysis identified size and surface ligands as the key factors affecting the antimicrobial efficacy of AgNMs.

The findings demonstrate the superior antibacterial activity of amphiphilic AgNCs compared to AgNPs against MDR Pseudomonas aeruginosa. This superiority stems from their enhanced interaction with bacterial membranes and increased ROS production. The results highlight the importance of considering the combined effects of size and surface ligands in designing effective AgNMs. The in vivo efficacy and biocompatibility of AgNCs suggest their potential as novel therapeutic agents for treating MDR bacterial infections. This study provides valuable insights into the design and development of advanced antimicrobial nanomaterials.

This research successfully demonstrated the enhanced antibacterial activity of amphiphilic silver nanoclusters against multidrug-resistant Pseudomonas aeruginosa. The unique properties of AgNCs, including their amphiphilic nature and nanocluster structure, resulted in superior performance compared to traditional AgNPs. The in vivo study confirmed the therapeutic potential and biocompatibility of these nanomaterials. This work opens new avenues for exploring AgNCs as promising candidates for combating multidrug-resistant bacterial infections. Further research could focus on optimizing AgNC synthesis, evaluating their efficacy against a broader range of MDR bacteria, and exploring their potential applications in various clinical settings.

The study primarily focused on Pseudomonas aeruginosa. Further research is needed to determine whether the observed effects are generalizable to other MDR bacterial species. While the in vivo study demonstrated efficacy in a mouse model, additional studies in larger animal models are needed before clinical translation. Long-term toxicity studies are also necessary to fully assess the biocompatibility and safety of AgNCs.