Lipophilic Ga Complex with Broad-Spectrum Antimicrobial Activity and the Ability to Overcome Gallium Resistance in both Pseudomonas aeruginosa and Staphylococcus aureus

Gallium: a decisive "Trojan Horse" against microorganisms

Controlling multidrug-resistant microorganisms (MRM) has a long history with the extensive and inappropriate use of antibiotics. At the cost of these drugs being scarce, new possibilities have to be explored to inhibit the growth of microorganisms. Thus, metallic compounds have shown to be promising as a viable alternative to contain pathogens resistant to conventional antimicrobials. Gallium (Ga3+) can be highlighted, which is an antimicrobial agent capable of disrupting the essential activities of microorganisms, such as metabolism, cellular respiration and DNA synthesis. It was observed that this occurs due to the similar properties between Ga3+ and iron (Fe3+), which is a fundamental ion for the correct functioning of bacterial activities. The mimetic effect performed by Ga3+ prevents iron transporters from distinguishing both ions and results in the substitution of Fe3+ for Ga3+ and in adverse metabolic disturbances in rapidly growing cells. This review focuses on analyzing the development of research involving Ga3+, elucidating the intracellular incorporation of the "Trojan Horse", summarizing the mechanism of interaction between gallium and iron and comparing the most recent and broad-spectrum studies using gallium-based compounds with antimicrobial scope.

Gallium-based metal-organic frameworks with antibacterial and anti-inflammatory properties for oral health protection

The fascial space of the oral and maxillofacial region contains loose connective tissues, which possess weak anti-infection ability and are often prone to infection, leading to acute suppurative inflammation and sepsis through blood. Although antibiotic use can reduce the probability of bacterial infections, owing to the emergence of antibiotic-resistant bacteria, the search for new antimicrobial drugs is imminent. Herein, we report a metal-organic framework (MOF) antibacterial material designed and synthesized with gallium (Ga) as the central atom, which possesses significant antibacterial, anti-inflammatory, and antioxidant effects. Our data suggested that GA-based MOFs (Ga-MOFs; 1 μg/mL) could sufficiently kill Porphyromonas gingivalis, Streptococcus pyogenes, and Staphylococcus aureus. Ga-MOFs exhibited a bactericidal effect against these three pathogens by disrupting biofilm formation, exopolysaccharide production, and bacterial membrane integrity. In addition, we found that 1 μg/mL of Ga-MOFs was not cytotoxic to human oral epithelial cell (HOEC) lines and it significantly reduced the adhesion of the three pathogens to HOEC. Ga-MOFs protect macrophages from excessive oxidative stress by scavenging excess intracellular reactive oxygen species and upregulating antioxidant gene levels, thereby enhancing cellular antioxidant defense. In addition, Ga-MOFs can promote the transformation of macrophages from the proinflammatory phenotype to the anti-inflammatory phenotype, thereby protecting oral health. Herein, novel Ga-MOF materials were chemically synthesized for therapeutic applications in oral infections, which provides new ideas for the development of novel nonantibiotic drugs to accelerate patient recovery.

Gallium-Containing Materials and Their Potential within New-Generation Titanium Alloys for Biomedical Applications

With the rising demand for implantable orthopaedic medical devices and the dominance of device-associated infections, extensive research into the development of novel materials has been prompted. Among these, new-generation titanium alloys with biocompatible elements and improved stiffness levels have received much attention. Furthermore, the development of titanium-based materials that can impart antibacterial function has demonstrated promising results, where gallium has exhibited superior antimicrobial action. This has been evidenced by the addition of gallium to various biomaterials including titanium alloys. Therefore, this paper aims to review the antibacterial activity of gallium when incorporated into biomedical materials, with a focus on titanium-based alloys. First, discussion into the development of new-generation Ti alloys that possess biocompatible elements and reduced Young's moduli is presented. This includes a brief review of the influence of alloying elements, processing techniques and the resulting biocompatibilities of the materials found in the literature. The antibacterial effect of gallium added to various materials, including bioglasses, liquid metals, and bioceramics, is then reviewed and discussed. Finally, a key focus is given to the incorporation of gallium into titanium systems for which the inherent mechanical, biocompatible, and antibacterial effects are reviewed and discussed in more detail, leading to suggestions and directions for further research in this area.

Comparative proteomics unveils the bacteriostatic mechanisms of Ga(III) on the regulation of metabolic pathways in Pseudomonas aeruginosa

Gallium has a long history as a chemotherapeutic agent. The mechanisms of action of Ga(III)-based anti-infectives are different from conventional antibiotics, which primarily result from the chemical similarities of Ga(III) with Fe(III) and substitution of gallium into iron-dependent biological pathways. However, more aspects of the molecular mechanisms of Ga(III) against human pathogens, especially the effects on bacterial metabolic processes, remain to be understood. Herein, by using conventional quantitative proteomics, we identified the protein changes of Pseudomonas aeruginosa (P. aeruginosa) in response to Ga(NO3)3 treatment. We show that Ga(III) exhibits bacteriostatic mode of action against P. aeruginosa through affecting the expressions of a number of key enzymes in the main metabolic pathways, including glycolysis, TCA cycle, amino acid metabolism, and protein and nucleic acid biosynthesis. In addition, decreased expressions of proteins associated with pathogenesis and virulence of P. aeruginosa were also identified. Moreover, the correlations between protein expressions and metabolome changes in P. aeruginosa upon Ga(III) treatment were identified and discussed. Our findings thus expand the understanding on the antimicrobial mechanisms of Ga(III) that shed light on enhanced therapeutic strategies. BIOLOGICAL SIGNIFICANCE: Mounting evidence suggest that the efficacy and resistance of clinical antibiotics are closely related to the metabolic homeostasis in bacterial pathogens. Ga(III)-based compounds have been repurposed as antibacterial therapeutic candidates against antibiotics resistant pathogens, and represent a safe and promising treatment for clinical human infections, while more thorough understandings of how bacteria respond to Ga(III) treatment are needed. In the present study, we provide evidences at the proteome level that indicate Ga(III)-induced metabolic perturbations in P. aeruginosa. We identified and discussed the interference of Ga(III) on the expressions and activities of enzymes in the main metabolic pathways in P. aeruginosa. In view of our previous report that the antimicrobial efficacy of Ga(III) could be modulated according to Ga(III)-induced metabolome changes in P. aeruginosa, our current analyses may provide theoretical basis at the proteome level for the development of efficient gallium-based therapies by exploiting bacterial metabolic mechanisms.

Development of Biocompatible Ga2(HPO4)3 Nanoparticles as an Antimicrobial Agent with Improved Ga Resistance Development Profile against Pseudomonas aeruginosa

Ga(III) can mimic Fe(III) in the biological system due to its similarities in charge and ionic radius to those of Fe(III) and can exhibit antimicrobial activity by disrupting the acquisition and metabolism of Fe in bacterial cells. For example, Ga(NO3)3 has been proven to be effective in treating chronic lung infections by Pseudomonas aeruginosa (P. aeruginosa) in cystic fibrosis patients in a recent phase II clinical trial. However, Ga(NO3)3 is an ionic compound that can hydrolyze to form insoluble hydroxides at physiological pH, which not only reduces its bioavailability but also causes potential renal toxicity when it is used as a systemic drug. Although complexion with suitable chelating agents has offered a varying degree of success in alleviating the hydrolysis of Ga(III), the use of nanotechnology to deliver this metallic ion should constitute an ultimate solution to all the above-mentioned problems. Thus far, the development of Ga-based nanomaterials as metalloantibiotics is an underexploited area of research. We have developed two different synthetic routes for the preparation of biocompatible Ga2(HPO4)3 NPs and shown that both the PVP- or PEG-coated Ga2(HPO4)3 NPs exhibit potent antimicrobial activity against P. aeruginosa. More importantly, such polymer-coated NPs do not show any sign of Ga-resistant phenotype development after 30 passes, in sharp contrast to Ga(NO3)3, which can rapidly develop Ga-resistant phenotypes of P. aeruginosa, indicating the potential of using Ga2(HPO4)3 NPs a new antimicrobial agent in place of Ga(NO3)3.

Harnessing the Dual Antimicrobial Mechanism of Action with Fe(8-Hydroxyquinoline)3 to Develop a Topical Ointment for Mupirocin-Resistant MRSA Infections

8-Hydroxyquinoline (8-hq) exhibits potent antimicrobial activity against Staphylococcus aureus (SA) bacteria with MIC = 16.0-32.0 µM owing to its ability to chelate metal ions such as Mn²⁺, Zn^2+, and Cu²⁺ to disrupt metal homeostasis in bacterial cells. We demonstrate that Fe(8-hq)₃, the 1:3 complex formed between Fe(III) and 8-hq, can readily transport Fe(III) across the bacterial cell membrane and deliver iron into the bacterial cell, thus, harnessing a dual antimicrobial mechanism of action that combines the bactericidal activity of iron with the metal chelating effect of 8-hq to kill bacteria. As a result, the antimicrobial potency of Fe(8-hq)₃ is significantly enhanced in comparison with 8-hq. Resistance development by SA toward Fe(8-hq)₃ is considerably delayed as compared with ciprofloxacin and 8-hq. Fe(8-hq)₃ can also overcome the 8-hq and mupirocin resistance developed in the SA mutant and MRSA mutant bacteria, respectively. Fe(8-hq)₃ can stimulate M1-like macrophage polarization of RAW 264.7 cells to kill the SA internalized in such macrophages. Fe(8-hq)₃ exhibits a synergistic effect with both ciprofloxacin and imipenem, showing potential for combination therapies with topical and systemic antibiotics for more serious MRSA infections. The in vivo antimicrobial efficacy of a 2% Fe(8-hq)₃ topical ointment is confirmed by the use of a murine model with skin wound infection by bioluminescent SA with a reduction of the bacterial burden by 99 ± 0.5%, indicating that this non-antibiotic iron complex has therapeutic potential for skin and soft tissue infections (SSTIs).

Broad-Spectrum Antimicrobial Activity of Ultrafine (BiO)2CO3 NPs Functionalized with PVP That Can Overcome the Resistance to Ciprofloxacin, AgNPs and Meropenem in Pseudomonas aeruginosa

Although it has no known biochemical role in living organisms, bismuth has been used to treat syphilis, diarrhea, gastritis and colitis for almost a century due to its nontoxic nature to mammalian cells. When prepared via a top-down sonication route from a bulk sample, bismuth subcarbonate (BiO)₂CO₃ nanoparticles (NPs) with an average size of 5.35 ± 0.82 nm exhibit broad-spectrum potent antibacterial activity against both the gram-positive and gram-negative bacteria including methicillin-susceptible Staphylococcus aureus (DSSA), methicillin-resistant Staphylococcus aureus (MRSA), drug-susceptible Pseudomonas aeruginosa (DSPA) and multidrug-resistant Pseudomonas aeruginosa (DRPA). Specifically, the minimum inhibitory concentrations (MICs) are 2.0 µg/mL against DSSA and MRSA and 0.75 µg/mL against DSPA and DRPA. In sharp contrast to ciprofloxacin, AgNPs and meropenem, (BiO)₂CO₃ NPs show no sign of developing Bi-resistant phenotypes after 30 consecutive passages. On the other hand, such NPs can readily overcome the resistance to ciprofloxacin, AgNPs and meropenem in DSPA. Finally, the combination of (BiO)₂CO₃ NPs and meropenem shows a synergistic effect with the fractional inhibitory concentration (FIC) index of 0.45.

Discovery of New Anti-MRSA Agents Based on Phenoxyethanol and Its Mechanism

Methicillin-resistant Staphylococcus aureus (MRSA) poses a severe threat to public health and safety. The discovery and development of novel anti-MRSA drugs with a new mode of action are a challenge. In this study, a class of novel aryloxyethyl propiolates and their homologues as anti-MRSA agents have been designed and synthesized based on phenoxyethanol, of which compound II-39 showed high inhibitory activity against MRSA with an MIC of 0.78 μg/mL and an MBC of 3.13 μg/mL, which was better than that of vancomycin. Compound II-39 could destroy the cell wall and cell membrane, inhibited the formation of a biofilm, and bound to the DNA of MRSA through the electrostatic and groove interaction. Proteomic and metabolomic studies revealed that compound II-39 affected multiple intracellular metabolic pathways of MRSA. Notably, compound II-39 could effectively inhibit the expression of CrtPQMN proteins and block the biosynthesis of virulence factor (staphyloxanthin). Thus, aryloxyethyl propiolates and their homologues are promising anti-MRSA agents with multiple targets.

Gallium containing bioactive materials: A review of anticancer, antibacterial, and osteogenic properties

The incorporation of gallium into bioactive materials has been reported to enhance osteogenesis, to influence blood clotting, and to induce anti-cancer and anti-bacterial activity. Gallium-doped biomaterials prepared by various techniques include melt-derived and sol-gel-derived bioactive glasses, calcium phosphate bioceramics, metals and coatings. In this review, we summarize the recently reported developments in antibacterial, anticancer, osteogenesis, and hemostasis properties of Ga-doped biomaterials and briefly outline the mechanisms leading to Ga biological effects. The key finding is that gallium addition to biomaterials has great potential for treating bone-related diseases since it can be efficiently transferred to the desired region at a controllable rate. Besides, it can be used as a potential substitute for antibiotics for the inhibition of infections during the initial and advanced phases of the wound healing process. Ga is also used as an anticancer agent due to the increased concentration of gallium around excessive cell proliferation (tumor) sites. Moreover, we highlight the possibility to design different therapeutic approaches aimed at increasing the efficiency of the use of gallium containing bioactive materials for multifunctional applications.

Strategies to prevent, curb and eliminate biofilm formation based on the characteristics of various periods in one biofilm life cycle

Biofilms are colonies of bacteria embedded inside a complicated self-generating intercellular. The formation and scatter of a biofilm is an extremely complex and progressive process in constant cycles. Once formed, it can protect the inside bacteria to exist and reproduce under hostile conditions by establishing tolerance and resistance to antibiotics as well as immunological responses. In this article, we reviewed a series of innovative studies focused on inhibiting the development of biofilm and summarized a range of corresponding therapeutic methods for biological evolving stages of biofilm. Traditionally, there are four stages in the biofilm formation, while we systematize the therapeutic strategies into three main periods precisely:(i) period of preventing biofilm formation: interfering the colony effect, mass transport, chemical bonds and signaling pathway of plankton in the initial adhesion stage; (ii) period of curbing biofilm formation:targeting several pivotal molecules, for instance, polysaccharides, proteins, and extracellular DNA (eDNA) via polysaccharide hydrolases, proteases, and DNases respectively in the second stage before developing into irreversible biofilm; (iii) period of eliminating biofilm formation: applying novel multifunctional composite drugs or nanoparticle materials cooperated with ultrasonic (US), photodynamic, photothermal and even immune therapy, such as adaptive immune activated by stimulated dendritic cells (DCs), neutrophils and even immunological memory aroused by plasmocytes. The multitargeted or combinational therapies aim to prevent it from developing to the stage of maturation and dispersion and eliminate biofilms and planktonic bacteria simultaneously.

Advantageous Reactivity of Unstable Metal Complexes: Potential Applications of Metal-Based Anticancer Drugs for Intratumoral Injections

Injections of highly cytotoxic or immunomodulating drugs directly into the inoperable tumor is a procedure that is increasingly applied in the clinic and uses established Pt-based drugs. It is advantageous for less stable anticancer metal complexes that fail administration by the standard intravenous route. Such hydrophobic metal-containing complexes are rapidly taken up into cancer cells and cause cell death, while the release of their relatively non-toxic decomposition products into the blood has low systemic toxicity and, in some cases, may even be beneficial. This concept was recently proposed for V(V) complexes with hydrophobic organic ligands, but it can potentially be applied to other metal complexes, such as Ti(IV), Ga(III) and Ru(III) complexes, some of which were previously unsuccessful in human clinical trials when administered via intravenous injections. The potential beneficial effects include antidiabetic, neuroprotective and tissue-regenerating activities for V(V/IV); antimicrobial activities for Ga(III); and antimetastatic and potentially immunogenic activities for Ru(III). Utilizing organic ligands with limited stability under biological conditions, such as Schiff bases, further enhances the tuning of the reactivities of the metal complexes under the conditions of intratumoral injections. However, nanocarrier formulations are likely to be required for the delivery of unstable metal complexes into the tumor.

Bi2O3 nanoparticles exhibit potent broad-spectrum antimicrobial activity and the ability to overcome Ag-, ciprofloxacin- and meropenem-resistance in P. aeruginosa: the next silver bullet of metal antimicrobials?

Antimicrobial resistance is a persistent threat to global public health. In order to combat the spread of pathogenic bacteria, numerous antimicrobial materials have been incorporated into wound dressings and medical devices such as implants and catheters. The most frequently utilized of these materials are Ag-salts and Ag-nanoparticles (AgNPs) due to their low minimum inhibitory concentrations (MICs) against common Gram-negative pathogenic bacteria such as P. aeruginosa. However, such Ag-based materials are limited to treating Gram-negative bacteria and prone to generating Ag-resistant phenotypes after only 7 consecutive exposures to these materials at a sub-inhibitory concentration. Here, we demonstrate α-Bi2O3 NPs as potential replacements for such materials, i.e., α-Bi2O3 NPs that exhibit potent broad-spectrum antibacterial activity (MIC = 0.75 μg mL-1 against P. aeruginosa; MIC = 2.5 μg mL-1 against S. aureus). Furthermore, these NPs are effective against Ag-resistant and carbapenem-resistant bacteria (MICs = 1.0 μg mL-1 and 1.25 μg mL-1, respectively) and also show a synergistic effect with meropenem (mero) in P. aeruginosa bacteria, allowing for the use of meropenem with smaller therapeutic doses (fractional inhibitory concentration = 0.45). Finally, unlike other materials that have been explored as effective antimicrobials, α-Bi2O3 NPs do not contribute to the development of Bi-resistant phenotypes after 30 passages of consecutive exposure to a sub-lethal dose of such NPs. Our results demonstrate that Bi-based materials represent a critical tool against multidrug resistant bacteria and require greater attention within the community. We anticipate this study to inspire broader investigation into the use of other metal oxides as antimicrobial materials, particularly those that limit the development of resistant phenotypes.

Advancement of Gallium and Gallium-Based Compounds as Antimicrobial Agents

With the abuse and misuse of antibiotics, antimicrobial resistance has become a challenging issue in the medical system. Iatrogenic and non-iatrogenic infections caused by multidrug-resistant (MDR) pathogens pose serious threats to global human life and health because the efficacy of traditional antibiotics has been greatly reduced and the resulting socio-economic burden has increased. It is important to find and develop non-antibiotic-dependent antibacterial strategies because the development of new antibiotics can hardly keep pace with the emergence of resistant bacteria. Gallium (III) is a multi-target antibacterial agent that has an excellent antibacterial activity, especially against MDR pathogens; thus, a gallium (III)-based treatment is expected to become a new antibacterial strategy. However, some limitations of gallium ions as antimicrobials still exist, including low bioavailability and explosive release. In recent years, with the development of nanomaterials and clathrates, the progress of manufacturing technology, and the emergence of synergistic antibacterial strategies, the antibacterial activities of gallium have greatly improved, and the scope of application in medical systems has expanded. This review summarizes the advancement of current optimization for these key factors. This review will enrich the knowledge about the efficiency and mechanism of various gallium-based antibacterial agents and provide strategies for the improvement of the antibacterial activity of gallium-based compounds.

Promising Antifungal and Antibacterial Agents Based on 5-Aryl-2,2'-bipyridines and Their Heteroligand Salicylate Metal Complexes: Synthesis, Bioevaluation, Molecular Docking

A series of new 5-aryl-2,2'-bipyridines and their (polyfluoro)salicylate complexes of Cu(II), Co(II) and Mn(II) were synthesized. Their antimicrobial activity was evaluated in vitro against six strains of Trichophytons, E. floccosum, M. canis, C. ablicans and Gram-negative bacteria N. gonorrhoeae. Among azo-ligands, Ph-bipy and Tol-bipy showed promising antifungal activity (minimum inhibitory concentration (MIC)<0.8-27 μM). Their antifungal action was found can be realized via binding Fe(III) ions. Tol-bipy suppressed growth of Gram-positive bacteria S. aureus, S. aureus MRSA and their monospecies biofilms (MIC 6-16 μM). Using molecular docking, the anti-staphylococcal action mechanism based on the inhibition of S. aureus DNA gyrase GyrB was proposed for the lead compounds. Among metal complexes, Cu(II) and Mn(II) complexes based on tetrafluorosalicylic acid and Tol-bipy or Ph-bipy had the high antifungal activity (MIC<0.24-32 μM). Mn(SalF₄ -2H)₂ (Tol-bipy)₂ ] suppressed the growth of seven Candida strains at MIC 12-24 μM. [Cu(Sal-2H)(Ph-bipy)] and [Cu(SalF₃ -2H)(Ph-bipy)₂ ] showed the promising anti-gonorrhoeae activity (MIC 4.2-5.2 μM). (Cu(SalF_n -2H)(Tol-bipy)₂ ], [Cu(SalF₄ -2H)(Ph-bipy)₂ ] and [Cu(SalF₃ -2H)(Ph-bipy)₂ ]) were found active against the bacteria of S. aureus, S. aureus MRSA and their biofilms (MIC 2.4-41.4 μM). The most active compounds were tested for toxicity in vitro against human embryonic kidney (HEK-293) cells and in vivo experiments with CD-1 mice.