Combining Gallium Protoporphyrin and Gallium Nitrate Enhances In Vitro and In Vivo Efficacy against Pseudomonas aeruginosa: Role of Inhibition of Bacterial Antioxidant Enzymes and Resultant Increase in Cytotoxic Reactive Oxygen Species

Thermodynamics of the Ga3+/ Fe3+ Competition in a Model of the Heme B-Containing Bacterial Catalase Active Center

Antibiotic resistance presents an enormous threat to human well-being due to the overconsumption and misuse of these essential drugs in recent years. A novel and intriguing path to overcoming the ever-pressing problem appears in the "Trojan horse" strategy exploiting bacteria's internalization systems and their exceptional capability to scavenge metal ions, iron in particular, from the surrounding media when evading the host organism. A promising candidate in this field is the abiogenic cation gallium─a ferric mimetic species, known to exert diverse effects, with its well-pronounced antibacterial activity attracting the attention of scientists in the past decade. In the study presented herewith, the computational chemistry methods, based on Density Functional Theory (DFT), are utilized in order to differentiate those outer factors contributing to gallium's ability to substitute the native ferric ion in the active site of the enzyme catalase. The characteristics of the surrounding media such as pH and solvent exposure, the composition of the protein shell, the nature of the metal, and different substrate molecules have been taken into account. The obtained results are interpreted in light of the experimentally reported observations and aim to contribute to deciphering this aspect of gallium's mechanism of antibacterial activity.

Uncovering the mysteries of bacterial cytochrome c oxidases: A review on structural and molecular insights for potential application

Cytochrome c oxidases are hemoproteins with a heme prosthetic group bound to the apoprotein. These complex enzymes are found embedded in the plasma membrane of the bacterial cells and play a vital role in the transfer of electrons from the electron transport chain to the oxygen molecule that acts as a terminal electron acceptor and gets reduced to water molecules. It helps establish a proton gradient across the plasma membrane by pumping hydrogen ions into the periplasmic space, generating adenosine triphosphate through oxidative phosphorylation. Bacteria have various cytochrome c oxidases based on the ecological niche that are differentially expressed with varying environmental conditions. Cytochrome c oxidases are made of different subunits with a distinct heme‑copper binuclear active site that catalyzes oxygen molecule reduction. Since these complex enzymes play a vital role in cellular respiration, the structure of cytochrome c oxidases remains conserved in many of the bacteria. Therefore, a detailed analysis of the structure of enzyme subunits, amino acid composition, and catalytic activity helps to design small molecules as drugs of clinical relevance for bacteria. The present review focuses on the structural details and molecular mechanisms such as proton pumping, electron transfer and the catalytic activity of oxygen reduction.

Efficacy and Possible Mechanism(s) of Action of Gallium Tetraphenylporphyrin Nanoparticles against HIV-TB Coinfection in an In Vitro Granuloma Structure Model

Coinfection of Mycobacterium tuberculosis (Mtb) and human immunodeficiency virus-1 (HIV) is a significant public health concern. Treatment is challenging due to prolonged duration of therapy and drug interactions between antiretroviral therapy (ART) and anti-TB drugs. Noniron gallium meso-tetraphenyl porphyrin (GaTP), a heme mimetic, has shown broad antimicrobial activity. Here, we investigated the efficacy of nanoparticle encapsulating GaTP (GaNP) for the treatment of HIV and Mtb coinfection or single infection in in vitro granuloma structures. GaNP significantly reduced viable Mtb within primary human in vitro granuloma structures infected with Mtb H37Rv-lux and significantly reduced levels of HIV in CD4+ T cells infected with the virus axenically. Similarly, GaNP exhibited significant antimicrobial activity against HIV/Mtb-coinfected granuloma structures created in vitro, which contain the primary immune cells seen in human TB granulomas, including CD4+ T cells and macrophages, as assessed by a luciferase assay for Mtb and p24 ELISA for HIV detection. Furthermore, mechanistic studies revealed that GaTP increases the level of reactive oxygen species and inhibits catalase in Mtb. A significant increase in Mtb nitrate reductase activity was also observed when Mtb was incubated with GaTP and sodium nitrate. Overall, increased oxidative stress and nitrite levels induced by GaTP are consistent with the possibility that GaTP inhibits Mtb aerobic respiration, which leads to incomplete O2 reduction and a shift to respiration using exogenous NO3. These cumulative data continue to support the potential for developing the noniron heme analog GaTP and its nanoparticle GaNP as new therapeutic approaches for the treatment of HIV/Mtb coinfection.

Emerging strategies to target virulence in Pseudomonas aeruginosa respiratory infections

Pseudomonas aeruginosa is an opportunistic pathogen that is responsible for infections in people living with chronic respiratory conditions, such as cystic fibrosis (CF) and non-CF bronchiectasis (NCFB). Traditionally, in people with chronic respiratory disorders, P. aeruginosa infection has been managed with a combination of inhaled and intravenous antibiotic therapies. However, due in part to the prolonged use of antibiotics in these people, the emergence of multi-drug resistant P. aeruginosa strains is a growing concern. The development of anti-virulence therapeutics may provide a new means of treating P. aeruginosa lung infections whilst also combatting the AMR crisis, as these agents are presumed to exert reduced pressure for the emergence of drug resistance as compared to antibiotics. However, the pipeline for developing anti-virulence therapeutics is poorly defined, and it is currently unclear as to whether in vivo and in vitro models effectively replicate the complex pulmonary environment sufficiently to enable development and testing of such therapies for future clinical use. Here, we discuss potential targets for P. aeruginosa anti-virulence therapeutics and the effectiveness of the current models used to study them. Focus is given to the difficulty of replicating the virulence gene expression patterns of P. aeruginosa in the CF and NCFB lung under laboratory conditions and to the challenges this poses for anti-virulence therapeutic development.

pH-mediated potentiation of gallium nitrate against Pseudomonas aeruginosa

The emergence of multidrug-resistant Pseudomonas aeruginosa isolates is a growing concern for public health, necessitating new therapeutic strategies. Gallium nitrate [Ga(NO3)3], a medication for cancer-related hypercalcemia, has attracted great attention due to its ability to inhibit P. aeruginosa growth and biofilm formation by disrupting iron metabolism. However, the antibacterial efficacy of Ga(NO3)3 is not always satisfactory. It is imperative to investigate the factors that affect the bactericidal effects of Ga(NO3)3 and to identify new ways to enhance its efficacy. This study focused on the impact of pH on P. aeruginosa resistance to Ga(NO3)3, along with the underlying mechanism. The results indicate that acidic conditions could increase the effectiveness of Ga(NO3)3 against P. aeruginosa by promoting the production of pyochelin and gallium uptake. Subsequently, using glutamic acid, a clinically compatible acidic amino acid, the pH was significantly lowered and enhanced the bactericidal and inhibitory efficacy of Ga(NO3)3 against biofilm formation by P. aeruginosa, including a reference strain PA14 and several multidrug-resistant clinical isolates. Furthermore, we used an abscess mouse model to evaluate this combination in vivo; the results show that the combination of glutamic acid and Ga(NO3)3 significantly improved P. aeruginosa clearance. Overall, the present study demonstrates that acidic conditions can increase the sensitivity of P. aeruginosa to Ga(NO3)3. Combining glutamic acid and Ga(NO3)3 is a potential strategy for the treatment of P. aeruginosa infections.

Antimicrobial Activity of Gallium(III) Compounds: Pathogen-Dependent Targeting of Multiple Iron/Heme-Dependent Biological Processes

Metals play vital roles in biological systems, with iron/heme being essential for cellular and metabolic functions necessary for survival and/or virulence in many bacterial pathogens. Given the rise of bacterial resistance to current antibiotics, there is an urgent need for the development of non-toxic and novel antibiotics that do not contribute to resistance to other antibiotics. Gallium, which mimics iron, has emerged as a promising antimicrobial agent, offering a novel approach to combat bacterial infections. Gallium does not have any known functions in biological systems. Gallium exerts its effects primarily by replacing iron in redox enzymes, effectively inhibiting bacterial growth by targeting multiple iron/heme-dependent biological processes and suppressing the development of drug resistance. The aim of this review is to highlight recent findings on the mechanisms of action of gallium and provide further insights into the development of gallium-based compounds. Understanding the mechanisms underlying gallium's biological activities is crucial for designing drugs that enhance their therapeutic therapies while minimizing side effects, offering promising avenues for the treatment of infectious diseases.

Implementation of Three Gallium-Based Complexes in the "Trojan Horse" Antibacterial Strategy against A. baumannii: A DFT Approach

Microorganisms of the ESKAPE group pose an enormous threat to human well-being, thus requiring a multidisciplinary approach for discovering novel drugs that are not only effective but utilize an innovative mechanism of action in order to decrease fast developing resistance. A promising but still hardly explored implementation in the "Trojan horse" antibacterial strategy has been recognized in gallium, an iron mimicry species with no known function but exerting a bacteriostatic/bactericidal effect against some representatives of the group. The study herewith focuses on the bacterium A. baumannii and its siderophore acinetobactin in its two isomeric forms depending on the acidity of the medium. By applying the powerful tools of the DFT approach, we aim to delineate those physicochemical characteristics that are of great importance for potentiating gallium's ability to compete with the native ferric cation for binding acinetobactin such as pH, solvent exposure (dielectric constant of the environment), different metal/siderophore ratios, and complex composition. Hence, the provided results not only furnish some explanation of the positive effect of three Ga3+-based anti-infectives in terms of metal cation competition but also shed light on reported in vitro and in vivo observations at a molecular level in regard to gallium's antibacterial effect against A. baumannii.

Balancing Bioresponsive Biofilm Eradication and Guided Tissue Repair via Pro-Efferocytosis and Bidirectional Pyroptosis Regulation during Implant Surgery

There is an increasingly growing demand to balance tissue repair guidance and opportunistic infection (OI) inhibition in clinical implant surgery. Herein, we developed a nanoadjuvant for all-stage tissue repair guidance and biofilm-responsive OI eradication via in situ incorporating Cobaltiprotoporphyrin (CoPP) into Prussian blue (PB) to prepare PB-CoPP nanozymes (PCZs). Released CoPP possesses a pro-efferocytosis effect for eliminating apoptotic and progressing necrotic cells in tissue trauma, thus preventing secondary inflammation. Once OIs occur, PCZs with switchable nanocatalytic capacity can achieve bidirectional pyroptosis regulation. Once reaching the acidic biofilm microenvironment, PCZs possess peroxidase (POD)-like activity that can generate reactive oxygen species (ROS) to eradicate bacterial biofilms, especially when synergized with the photothermal effect. Furthermore, generated ROS can promote macrophage pyroptosis to secrete inflammatory cytokines and antimicrobial proteins for biofilm eradication in vivo. After eradicating the biofilm, PCZs possess catalase (CAT)-like activity in a neutral environment, which can scavenge ROS and inhibit macrophage pyroptosis, thereby improving the inflammatory microenvironment. Briefly, PCZs as nanoadjuvants feature the capability of all-stage tissue repair guidance and biofilm-responsive OI inhibition that can be routinely performed in all implant surgeries, providing a wide range of application prospects and commercial translational value.

In Silico Analysis of the Ga3+/Fe3+ Competition for Binding the Iron-Scavenging Siderophores of P. aeruginosa-Implementation of Three Gallium-Based Complexes in the "Trojan Horse" Antibacterial Strategy

The emergence of multidrug-resistant (MDR) microorganisms combined with the ever-draining antibiotic pipeline poses a disturbing and immensely growing public health challenge that requires a multidisciplinary approach and the application of novel therapies aimed at unconventional targets and/or applying innovative drug formulations. Hence, bacterial iron acquisition systems and bacterial Fe2+/3+-containing enzymes have been identified as a plausible target of great potential. The intriguing "Trojan horse" approach deprives microorganisms from the essential iron. Recently, gallium's potential in medicine as an iron mimicry species has attracted vast attention. Different Ga3+ formulations exhibit diverse effects upon entering the cell and thus supposedly have multiple targets. The aim of the current study is to specifically distinguish characteristics of great significance in regard to the initial gallium-based complex, allowing the alien cation to effectively compete with the native ferric ion for binding the siderophores pyochelin and pyoverdine secreted by the bacterium P. aeruginosa. Therefore, three gallium-based formulations were taken into consideration: the first-generation gallium nitrate, Ga(NO3)3, metabolized to Ga3+-hydrated forms, the second-generation gallium maltolate (tris(3-hydroxy-2-methyl-4-pyronato)gallium), and the experimentally proven Ga carrier in the bloodstream-the protein transferrin. We employed a reliable in silico approach based on DFT computations in order to understand the underlying biochemical processes that govern the Ga3+/Fe3+ rivalry for binding the two bacterial siderophores.

Gallium-based metal-organic frameworks loaded with antimicrobial peptides for synergistic killing of drug-resistant bacteria

Increased antibiotic resistance has made bacterial infections a global concern, which requires novel non-antibiotic-dependent antibacterial strategies to address the menace. Antimicrobial peptides (AMPs) are a promising antibiotic alternative, whose antibacterial mechanism is mainly to destroy the membrane of bacteria. Gallium ions exhibit an antibacterial effect by interfering with the iron metabolism of bacteria. With the rapid development of nanotechnology, it is worth studying the potential of gallium-AMP-based nanocomposites for treating bacterial infections. Herein, novel gallium-based metal-organic frameworks (MOFs) were synthesized at room temperature, followed by in situ loading of the model AMP melittin. The obtained nanocomposites exhibited stronger antibacterial activity than pure MEL and gallium ions, achieving the effects of "one plus one is greater than two". Moreover, the nanocomposites showed favorable biocompatibility and accelerated healing of a wound infected by methicillin-resistant Staphylococcus aureus by down-regulation of inflammatory cytokines IL-6 and TNF-α. This work presents an innovative antibacterial strategy to overcome the antibiotic resistance crisis and expand the application of AMPs.

Gallium Nitrate Enhances Antimicrobial Activity of Colistin against Klebsiella pneumoniae by Inducing Reactive Oxygen Species Accumulation

Klebsiella pneumoniae, a pathogen of critical clinical concern, urgently demands effective therapeutic options owing to its drug resistance. Polymyxins are increasingly regarded as a last-line therapeutic option for the treatment of multidrug-resistant (MDR) Gram-negative bacterial infections. However, polymyxin resistance in K. pneumoniae is an emerging issue. Here, we report that gallium nitrate (GaNt), an antimicrobial candidate, exhibits a potentiating effect on colistin against MDR K. pneumoniae clinical isolates. To further confirm this, we investigated the efficacy of combined GaNt and colistin in vitro using spot dilution and rapid time-kill assays and growth curve inhibition tests and in vivo using a murine lung infection model. The results showed that GaNt significantly increased the antimicrobial activity of colistin, especially in the iron-limiting media. Mechanistic studies demonstrated that bacterial antioxidant activity was repressed by GaNt, as revealed by RNA sequencing (RNA-seq), leading to intracellular accumulation of reactive oxygen species (ROS) in K. pneumoniae, which was enhanced in the presence of colistin. Therefore, oxidative stress induced by GaNt and colistin augments the colistin-mediated killing of wild-type cells, which can be abolished by dimethyl sulfoxide (DMSO), an effective ROS scavenger. Collectively, our study indicates that GaNt has a notable impact on the antimicrobial activity of colistin against K. pneumoniae, revealing the potential of GaNt as a novel colistin adjuvant to improve the treatment outcomes of bacterial infections. IMPORTANCE This study aimed to determine the antimicrobial activity of GaNt combined with colistin against Klebsiella pneumoniae in vitro and in vivo. Our results suggest that by combining GaNt with colistin, antioxidant activity was suppressed and reactive oxygen species accumulation was induced in bacterial cells, enhancing antimicrobial activity against K. pneumoniae. We found that GaNt functioned as an antibiotic adjuvant when combined with colistin by inhibiting the growth of multidrug-resistant K. pneumoniae. Our study provides insight into the use of an adjuvant to boost the antibiotic potential of colistin for treating infections caused by multidrug-resistant K. pneumoniae.

Combination Therapy with Gallium Protoporphyrin and Gallium Nitrate Exhibits Enhanced Antimicrobial Activity In Vitro and In Vivo against Methicillin-Resistant Staphylococcus aureus

There is a major need for the development of new therapeutics to combat antibiotic-resistant Staphylococcus aureus. Recently, gallium (Ga)-based complexes have shown promising antimicrobial effects against various bacteria, including multidrug-resistant organisms, by targeting multiple heme/iron-dependent metabolic pathways. Among these, Ga protoporphyrin (GaPP) inhibits bacterial growth by targeting heme pathways, including aerobic respiration. Ga(NO3)3, an iron mimetic, disrupts elemental iron pathways. Here, we demonstrate the enhanced antimicrobial activity of the combination of GaPP and Ga(NO3)3 against methicillin-resistant S. aureus (MRSA) under iron-limited conditions, including small colony variants (SCV). This therapy demonstrated significant antimicrobial activity without inducing slow-growing SCV. We also observed that the combination of GaPP and Ga(NO3)3 inhibited the MRSA catalase but not above that seen with Ga(NO3)3 alone. Neither GaPP nor Ga(NO3)3 alone or their combination inhibited the dominant superoxide dismutase expressed (SodA) under the iron-limited conditions examined. Intranasal administration of the combination of the two compounds improved drug biodistribution in the lungs compared to intraperitoneal administration. In a murine MRSA lung infection model, we observed a significant increase in survival and decrease in MRSA lung CFUs in mice that received combination therapy with intranasal GaPP and Ga(NO3)3 compared to untreated control or mice receiving GaPP or Ga(NO3)3 alone. No drug-related toxicity was observed as assessed histologically in the spleen, lung, nasal cavity, and kidney for both single and repeated doses of 10 mg Ga /Kg of mice over 13 days. Our results strongly suggest that GaPP and Ga(NO3)3 in combination have excellent synergism and potential to be developed as a novel therapy for infections with S. aureus.

Silver Nanoparticles Modified with Polygonatum sibiricum Polysaccharide Improve Biocompatibility and Infected Wound Bacteriostasis

Silver nanoparticles (AgNPs) exhibit strong antibacterial activity and do not easily induce drug resistance; however, the poor stability and biocompatibility in solution limit their widespread application. In this study, AgNPs were modified with Polygonatum sibiricum Polysaccharide (PSP) to synthesize PSP@AgNPs with good stability, biocompatibility, and antibacterial activity. When PSP@AgNP synthesis was performed under a reaction time of 70 min, a reaction temperature of 35 °C, and an AgNO₃-to-PSP volume ratio of 1:1, the synthesized PSP@AgNPs were more regular and uniform than AgNPs, and their particle size was around 10 nm. PSP@AgNPs exhibited lower cytotoxicity and hemolysis, and stronger bacteriostatic activity. PSP@AgNPs damage the integrity and internal structure of cells, resulting in the leakage of intracellular nucleic acids and proteins. The rate of cell membrane damage in Escherichia coli and Staphylococcus aureus treated with PSP@AgNPs increased by 38.52% and 43.75%, respectively, compared with that of AgNPs. PSP@AgNPs inhibit the activities of key enzymes related to antioxidant, energy and substance metabolism in cells. The inhibitory effects on the activities of superoxide dismutase (SOD), catalase (CAT), adenosine triphosphate enzyme (ATPase), malate dehydrogenase (MDH), and succinate dehydrogenase (SDH) in E. coli and S. aureus cells were significantly higher than those of AgNPs. In addition, compared with AgNPs, PSP@AgNPs promote faster healing of infected wounds. Therefore, PSP@AgNPs represent potential antibacterial agents against wound infections.