Golden Nanoclusters: Unlocking the Power of Gold to Fight Superbugs

Pseudomonas aeruginosa is extremely opportunistic and can cause serious infections in hospital settings, especially in immunocompromised people. The rise of multidrug-resistant (MDR) P. aeruginosa infections has become a significant concern in the global public health community. Over time, with exposure to various antibiotics, strains of P. aeruginosa have acquired multiple resistance mechanisms, leading to MDR strains. These resistances can be due to mutations or the acquisition of resistance genes via horizontal gene transfer mechanisms. MDR P. aeruginosa is a leading cause of hospital-acquired (nosocomial) infections, especially in intensive care units. It can cause a wide range of infections, including pneumonia, bloodstream infections, urinary tract infections, and wound infections. As resistance grows, the number of effective antibiotics dwindles. Some MDR Pseudomonas aeruginosa strains are resistant to almost all available antibiotics, leaving very few treatment options. In some cases, healthcare providers have to resort to using last resort, more toxic antibiotics such as colistin, or combinations of antibiotics, hoping for a synergistic effect. Infections caused by MDR Pseudomonas aeruginosa are associated with prolonged hospital stays, higher medical costs, and increased mortality rates. The mortality rate for bloodstream infections, for instance, can be particularly high. Efforts to curb the spread of MDR Pseudomonas aeruginosa and other resistant organisms must be comprehensive and collaborative. A new study published in the Angewandte Chemie International Edition Journal, led by Professor Mingdi Yan and Professor Olof Ramström from the University of Massachusetts Lowell and conducted by Dr. William Ndugire, Dang Truong, N G Hasitha Raviranga, and Jingzhe Lao, explored the potential of gold nanoparticles (AuNPs), specifically gold nanoclusters (AuNCs), as an alternative strategy to combat antimicrobial resistance (AMR). The authors reported that thiourea (TU) can initiate the antimicrobial activity of AuNPs by converting inactive AuNPs into antimicrobial Au^I species. Both Au^I and TU historically were used in medicine, primarily for the treatment of inflammatory arthritis and hyperthyroidism, respectively. TU exhibits low cytotoxicity and can also act as a reactive oxygen species scavenger, making it a promising candidate for enhancing the antibacterial properties of AuNPs.

The researchers designed a suitable ligand for passivating AuNCs, choosing D-maltose (Mal) as the model system. The choice of carbohydrate functionalization was driven by its potential to modulate cellular uptake and reduce the toxicity of nanomaterials. D-Maltose, in particular, is taken up through dedicated transport channels in Gram-negative bacteria such as the well-characterized maltodextrin/maltose transport system of E. coli. The results demonstrated that Mal-functionalized AuNCs (AuNC-Mal) were non-toxic to both mammalian and prokaryotic cells. However, the addition of TU activated the antibacterial activity of AuNC-Mal, leading to significant antibacterial effects against a range of bacteria, including MDR P. aeruginosa clinical isolates.

According to the authors, the synthesis of AuNC-Mal involved a ligand exchange and core etching process using a thiolated Mal under acidic conditions. This new method resulted in the formation of AuNCs with a size range of approximately 1.8 nm. Electrospray ionization mass spectrometry (ESI-MS) confirmed the presence of various AuNC sizes, with a major peak corresponding to [Au₂₂(MalS)₁₅(PPh₃)Cl₃]⁺. Proton nuclear magnetic resonance spectroscopy (¹H NMR) further supported the successful synthesis of AuNC-Mal.

The authors’ findings also highlighted the mechanism of action of AuNC-Mal/TU. They observed that TU played a crucial role in preventing the oxidation of Au^I to Au^III, thereby maintaining the gold species in its more active Au^I state. This is important because Au^I is known to be more effective against bacteria than Au^III. In addition, TU increased the accumulation of Au within bacterial cells, contributing to its antibacterial activity. Furthermore, the authors investigated the impact of AuNC-Mal/TU on bacterial membranes, specifically the outer membrane (OM) of Gram-negative bacteria, which is known to act as a barrier against many antibiotics. They found that AuNC-Mal/TU readily penetrated the OM, and the antibacterial activity was not affected by membrane-compromising agents such as colistin. This suggests that the mechanism of action of AuNC-Mal/TU does not rely on OM disruption. The study also explored the role of AuNC-Mal/TU in inhibiting thioredoxin reductase (TrxR), an enzyme critical for bacterial redox homeostasis. The findings revealed that AuNC-Mal/TU effectively inhibited bacterial TrxR, which is consistent with previous reports on the antimicrobial activity of certain gold drugs such as auranofin. Moreover, the researchers examined the impact of copper ions (Cu^I and Cu^II) on the activity of AuNC-Mal/TU. While Cu^I complexes antagonized the activity of AuNC-Mal/TU, Cu^II had no significant effect. This suggests that AuNC-Mal/TU may bear similarity to Cu^I in their antimicrobial modes of action.

The activity of AuNC-Mal in combination with TU against methicillin-resistant Staphylococcus aureus (MRSA) is another significant aspect of the research. MRSA is a particularly dangerous strain of Staphylococcus aureus bacteria that has developed resistance to many common antibiotics, including methicillin. It poses a significant threat to public health due to its ability to cause severe infections in healthcare settings and in the community. The authors demonstrated that AuNC-Mal/TU had remarkable antimicrobial activity against MRSA, underlining its potential as a novel treatment option for combating this challenging pathogen. MRSA strains are notorious for their resistance to conventional antibiotics, making infections difficult to treat and often leading to severe complications. The emergence of multidrug-resistant MRSA strains has heightened the urgency to develop alternative therapeutic approaches. AuNC-Mal, when tested on its own, exhibited limited antimicrobial activity against MRSA, with a high minimum inhibitory concentration (MIC). However, the addition of TU significantly enhanced the antibacterial effect of AuNC-Mal against MRSA, reducing the MIC to levels that suggest potent antimicrobial action. This is a promising finding, as it indicates that AuNC-Mal/TU could potentially be used to combat MRSA infections effectively. The combination of AuNC-Mal and TU appears to work through multiple modes of action, inhibiting critical bacterial processes and disrupting the defense mechanisms that MRSA employs to resist antibiotics. These modes of action include inhibiting TrxR, depleting ATP, and interfering with copper regulation within the bacterial cells. Such a multifaceted approach is particularly valuable in combating antibiotic-resistant pathogens like MRSA, as it reduces the likelihood of bacteria developing resistance to this novel treatment. Furthermore, AuNC-Mal/TU’s ability to eradicate Staphylococcus biofilms is a crucial aspect of its antimicrobial activity. Biofilms are complex communities of bacteria encased in a protective matrix, making them highly resistant to antibiotics and the host immune response. The successful eradication of Staphylococcus biofilms by AuNC-Mal/TU suggests that this combination has the potential to address persistent MRSA infections, which are often associated with biofilm formation.

The potential of AuNC-Mal/TU to overcome colistin resistance is an exciting prospect in the field of antimicrobial research. Colistin, a last-resort antibiotic, has been used to combat multidrug-resistant bacteria, including those resistant to other antibiotics. However, the emergence of colistin-resistant strains in bacteria such as Klebsiella pneumoniae and Pseudomonas aeruginosa, has raised concerns about the effectiveness of this antibiotic. Researchers are actively seeking alternative strategies to combat these highly resistant pathogens, and AuNC-Mal/TU presents a promising avenue. Colistin resistance often arises due to modifications in the bacterial outer membrane, which reduce the drug’s ability to bind to its target and disrupt the cell membrane. AuNC-Mal, when used alone, may have limited antibacterial activity against colistin-resistant strains. However, as shown in previous studies against MRSA, the addition of TU significantly enhances the antimicrobial effect of AuNC-Mal. This synergistic action may allow AuNC-Mal/TU to overcome resistance mechanisms that hinder colistin’s efficacy. The multifaceted mechanism of action of AuNC-Mal/TU may reduce the likelihood of resistance development in colistin-resistant strains. By targeting multiple bacterial processes simultaneously, it becomes more difficult for bacteria to adapt and develop resistance. Moreover, AuNC-Mal/TU could be used in combination with existing antibiotics, including colistin, to enhance their effectiveness against resistant strains. This combination therapy approach may provide a more potent and sustainable solution to combat colistin resistance.

In terms of mammalian toxicity, AuNC-Mal/TU demonstrated low toxicity to human alveolar epithelial cancer cells (A549) and mouse fibroblast cells (NIH/3T3). The selectivity of AuNC-Mal/TU for bacterial cells over mammalian cells was evident, with AuNC-Mal/TU being significantly less toxic to mammalian cells than to P. aeruginosa PAO1. In summary, the new study by Professor Mingdi Yan and her research group represents a significant advancement in the development of alternative strategies to combat antimicrobial resistance. The use of gold nanoclusters functionalized with D-maltose and activated by thiourea provides a novel and promising approach to address multidrug-resistant bacterial infections, including those that have emerged as a result of the COVID-19 pandemic. The findings also offered valuable insights into the mechanisms underlying the antibacterial activity of gold compounds and provided a foundation for further investigations and potential therapeutic applications. In a statement to Medicine Innovates series, lead author, Dr. William Ndugire, said “Exploring the multifaceted potential of AuNC-Mal/TU is an exhilarating journey into the world of antimicrobial research. Beyond its remarkable efficacy against bacteria, this innovative approach holds the promise of combating a broader spectrum of microbial threats. Notably, the FDA-approved gold drug auranofin has emerged as a compelling candidate in the fight against SARS-CoV-2, shedding light on the diverse applications of AuNC-Mal/TU in tackling infectious diseases. This synergy between nanomedicine and established therapeutics opens new avenues for addressing the ever-evolving challenges posed by infectious agents in our modern world.”