The gallium(III)–salicylidene acylhydrazide complex shows synergistic anti-biofilm effect and inhibits toxin production by Pseudomonas aeruginosa

Antibiofilm Activity of Gallium(III) Complexed Anionic Polymers in Combination with Antibiotics

Pseudomonas aeruginosa (P. aeruginosa) is a life-threatening pathogen associated with multiantibiotic resistance, which is largely caused by its strong ability to form biofilms. Recent research has revealed that gallium (III) shows an activity against the biofilm of P. aeruginosa by interfering with Fe metabolism. The antibacterial activity of the combination of Ga³⁺ ion and antibiotic rifampicin (RMP) against P. aeruginosa PAO1 is investigated. An anionic polymer poly{{2-[(2-methylprop-2-enoyl)oxy]ethyl}phosphonic acid} (PDMPOH) is exploited to form complexes (GaPD) with Ga³⁺ . The GaPD complexes act as a carrier of Ga³⁺ and release Ga³⁺ via enzymatic degradation by bacterial lipases. GaPD is found to damage the outer membrane, leading to enhanced cellular uptake of RMP and Ga³⁺ due to increased outer membrane permeability, which inhibits the RNA polymerase and interferes with Fe metabolism. The antibiofilm activity and biocompatibility of the GaPD system offer a promising treatment option for P. aeruginosa biofilm-related infections.

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

Antibiotic resistance (AR) necessitates the discovery of new antimicrobials with alternative mechanisms of action to those employed by conventional antibiotics. One such strategy utilizes Ga³⁺ to target iron metabolism, a critical process for survival. Still, Ga-based therapies are generally ineffective against Gram-positive bacteria and promote Ga resistance. In response to these drawbacks, we report a lipophilic Ga complex, [Ga₂L₃(bpy)₂] (L = 2,2'-bis(3-hydroxy-1,4-naphthoquinone; bpy = 2,2'-bipyridine)), effective against drug-resistant Pseudomonas aeruginosa (DRPA; minimum inhibitory concentration, MIC = 10 μM = 14.8 μg/mL) and methicillin-resistant Staphylococcus aureus (MRSA, MIC = 100 μM = 148 μg/mL) without iron-limited conditions. Importantly, [Ga₂L₃(bpy)₂] shows noticeably delayed and decreased resistance in both MRSA and DRPA, with only 8× MIC in DRPA and none in MRSA after 30 passages. This is likely due to the dual mode of action afforded by Ga (disruption of iron metabolism) and the ligand (reactive oxygen species production). Overall, [Ga₂L₃(bpy)₂] demonstrates the utility of lipophilic metal complexes with multiple modes of action in combatting AR in Gram-positive and Gram-negative bacteria.

Ruthenium(IV) Complexes as Potential Inhibitors of Bacterial Biofilm Formation

With increasing antimicrobial resistance there is an urgent need for new strategies to control harmful biofilms. In this study, we have investigated the possibility of utilizing ruthenium(IV) complexes (H₃O)₂(HL1)₂[RuCl₆]·2Cl·2EtOH (1) and [RuCl₄(CH₃CN)₂](L³2)·H₂O (2) (where L1-2-hydroxymethylbenzimadazole, L³2-1,4-dihydroquinoxaline-2,3-dione) as effective inhibitors for biofilms formation. The biological activities of the compounds were explored using E. coli, S. aureus, P. aeruginosa PAO1, and P. aeruginosa LES B58. The new chloride ruthenium complexes were characterized by single-crystal X-ray diffraction analysis, Hirshfeld surface analysis, FT-IR, UV-Vis, magnetic and electrochemical (CV, DPV) measurements, and solution conductivity. In the obtained complexes, the ruthenium(IV) ions possess an octahedral environment. The intermolecular classical and rare weak hydrogen bonds, and π···π stacking interactions significantly contribute to structure stabilization, leading to the formation of a supramolecular assembly. The microbiological tests have shown complex 1 exhibited a slightly higher anti-biofilm activity than that of compound 2. Interestingly, electrochemical studies have allowed us to determine the relationship between the oxidizing properties of complexes and their biological activity. Probably the mechanism of action of 1 and 2 is associated with generating a cellular response similar to oxidative stress in bacterial cells.

Crystal structure of (E)-3-methoxy-N′-(1-(pyridin-2-yl)ethylidene)benzohydrazide, C15H15N3O2

C15H15N3O2, orthorhombic, Pca21 (no. 29), a = 7.9831(2) Å, b = 10.6486(3) Å, c = 15.7222(4) Å, V = 1336.53(6) Å3, Z = 4, R gt(F) = 0.0340, wR ref(F 2) = 0.0799, T = 100 K.

Intra-molecular 1,5-S⋯N σ-hole inter-action in (E)-N'-(pyridin-4-yl-methyl-idene)thio-phene-2-carbohydrazide

The title compound, C11H9N3OS, (I), crystallizes in the monoclinic space group P21/n. The mol-ecular conformation is nearly planar and features an intra-molecular chalcogen bond between the thio-phene S and the imine N atoms. Within the crystal, the strongest inter-actions between mol-ecules are the N-H⋯O hydrogen bonds, which organize them into inversion dimers. The dimers are linked through short C-H⋯N contacts and are stacked into layers propagating in the (001) plane. The crystal structure features π-π stacking between the pyridine aromatic ring and the azomethine double bond. The calculated energies of pairwise inter-molecular inter-actions within the stacks are considerably larger than those found for the inter-actions between the layers.

Identification and Characterization of a Metalloprotein Involved in Gallium Internalization in Pseudomonas aeruginosa

Gallium nitrate (Ganite) is a potential drug for the treatment of Pseudomonas aeruginosa infection. CRISPR/Cas9-based gene mutagenesis studies reveal that siderophore pyochelin-facilitated uptake and an ABC transporter are two major Ga³⁺ internalization pathways in Pseudomonas aeruginosa (P. aeruginosa). Crystal structures reveal that Ga³⁺ and Fe³⁺ occupy exactly the same metal site of HitA, a periplasmic iron-binding protein of the ABC transporter system. The study provides a molecular basis for Ga³⁺ internalization by P. aeruginosa and facilitates gallium-based antimicrobial drug development.

The salicylidene acylhydrazide INP0341 attenuates Pseudomonas aeruginosa virulence in vitro and in vivo

Pseudomonas aeruginosa is an opportunistic pathogen that can be very hard to treat because of high resistance to different antibiotics and alternative treatment regimens are greatly needed. An alternative or a complement to traditional antibiotic is to inhibit virulence of the bacteria. The salicylidene acylhydrazide, INP0341, belongs to a class of compounds that has previously been shown to inhibit virulence in a number of Gram-negative bacteria. In this study, the virulence blocking effect of INP0341 on P. aeruginosa was studied in vitro and in vivo. Two important and closely related virulence system were examined, the type III secretion system (T3SS) that translocates virulence effectors into the cytosol of the host cell to evade immune defense and facilitate colonization and the flagella system, needed for motility and biofilm formation. INP0341 was shown to inhibit expression and secretion of the T3SS toxin exoenzyme S (ExoS) and to prevent bacterial motility on agar plates and biofilm formation. In addition, INP0341 showed an increased survival of P. aeruginosa-infected mice. In conclusion, INP0341 attenuates P. aeruginosa virulence.

Biological study of the effect of water soluble [N-(2-hydroxybenzyl)-L-aspartato] gallium complexes on breast carcinoma and fibrosarcoma cells

Two water soluble gallium complexes described as [Ga(III)LCl], where L is the deprotonated form of N-2-hydroxybenzyl aspartic acid derivatives, were synthesized and characterized by (1)H NMR, (13)C NMR, FT-IR, mass spectrometry, and elemental analysis. The 2-(5-chloro-2-hydroxybenzylamino)succinic acid derivative (GS2) has been found to be a promising anticancer drug candidate. This compound was found to be more cytotoxic against human breast carcinoma MDA-MB231 and fibrosarcoma HT-1080 cell lines than the unsubstituted derivative and GaCl3. GS2 was able to induce apoptosis through downregulation of AKT phosphorylation, G2M arrest in cell cycle, and caspase 3/7 pathway. This gallium complex was found to induce an increase in mitochondrial ROS level in HT-1080 cells but not in MDA-MB231 cells. This suggests that the mechanism of action of GS2 would not be mediated by the drug-induced oxidative stress but probably by directly and indirectly inhibiting the AKT cell-signaling pathway.

Targeting the Type Three Secretion System in Pseudomonas aeruginosa

The injectisome type three secretion system (T3SS) is a major virulence factor in Pseudomonas aeruginosa. This bacterium is responsible for severe infections in immunosuppressed or cystic fibrosis patients and has become resistant to many antibiotics. Inhibitors of T3SS may therefore constitute an innovative therapeutic target. After a brief description of the T3SS and its regulation, this review presents strategies to inhibit T3SS-mediated toxicity and describes the main families of existing inhibitors. Over the past few years, 12 classes of small-molecule inhibitors and two types of antibody have been discovered and evaluated in vitro for their capacity to inhibit T3SS expression or function, and to protect host cells from T3SS-mediated cytotoxicity. While only one small molecule has been tested in vivo, a bifunctional antibody targeting both the translocation apparatus of the T3SS and a surface polysaccharide is currently in Phase II clinical trials.

Inhibition of Pseudomonas aeruginosa ExsA DNA-Binding Activity by N-Hydroxybenzimidazoles

The Pseudomonas aeruginosa type III secretion system (T3SS) is a primary virulence determinant and a potential target for antivirulence drugs. One candidate target is ExsA, a member of the AraC family of DNA-binding proteins required for expression of the T3SS. A previous study identified small molecules based on an N-hydroxybenzimidazole scaffold that inhibit the DNA-binding activity of several AraC proteins, including ExsA. In this study, we further characterized a panel of N-hydroxybenzimidazoles. The half-maximal inhibitory concentrations (IC50s) for the tested N-hydroxybenzimidazoles ranged from 8 to 45 μM in DNA-binding assays. Each of the N-hydroxybenzimidazoles protected mammalian cells from T3SS-dependent cytotoxicity, and protection correlated with reduced T3SS gene expression in a coculture infection model. Binding studies with the purified ExsA DNA-binding domain (i.e., lacking the amino-terminal self-association domain) confirmed that the activity of N-hydroxybenzimidazoles results from interactions with the DNA-binding domain. The interaction is specific, as an unrelated DNA-binding protein (Vfr) was unaffected by N-hydroxybenzimidazoles. ExsA homologs that control T3SS gene expression in Yersinia pestis, Aeromonas hydrophila, and Vibrio parahaemolyticus were also sensitive to N-hydroxybenzimidazoles. Although ExsA and Y. pestis LcrF share 79% sequence identity in the DNA-binding domain, differential sensitivities to several of the N-hydroxybenzimidazoles were observed. Site-directed mutagenesis based on in silico docking of inhibitors to the DNA-binding domain, and on amino acid differences between ExsA and LcrF, resulted in the identification of several substitutions that altered the sensitivity of ExsA to N-hydroxybenzimidazoles. Development of second-generation compounds targeted to the same binding pocket could lead to drugs with improved pharmacological properties.

The bacterial type III secretion system as a target for developing new antibiotics

Antibiotic resistance in pathogens requires new targets for developing novel antibacterials. The bacterial type III secretion system (T3SS) is an attractive target for developing antibacterials as it is essential in the pathogenesis of many Gram-negative bacteria. The T3SS consists of structural proteins, effectors, and chaperones. Over 20 different structural proteins assemble into a complex nanoinjector that punctures a hole on the eukaryotic cell membrane to allow the delivery of effectors directly into the host cell cytoplasm. Defects in the assembly and function of the T3SS render bacteria non-infective. Two major classes of small molecules, salicylidene acylhydrazides and thiazolidinones, have been shown to inhibit multiple genera of bacteria through the T3SS. Many additional chemically and structurally diverse classes of small molecule inhibitors of the T3SS have been identified as well. While specific targets within the T3SS of a few inhibitors have been suggested, the vast majority of specific protein targets within the T3SS remain to be identified or characterized. Other T3SS inhibitors include polymers, proteins, and polypeptides mimics. In addition, T3SS activity is regulated by its interaction with biologically relevant molecules, such as bile salts and sterols, which could serve as scaffolds for drug design.

Evaluation of Gallium Citrate Formulations against a Multidrug-Resistant Strain of Klebsiella pneumoniae in a Murine Wound Model of Infection

Skin and soft tissue infections (SSTIs) are a common occurrence in health care facilities with a heightened risk for immunocompromised patients. Klebsiella pneumoniae has been increasingly implicated as the bacterial agent responsible for SSTIs, and treatment can be challenging as more strains become multidrug resistant (MDR). Therefore, new treatments are needed to counter this bacterial pathogen. Gallium complexes exhibit antimicrobial activity and are currently being evaluated as potential treatment for bacterial infections. In this study, we tested a topical formulation containing gallium citrate (GaCi) for the treatment of wounds infected with K. pneumoniae. First, the MIC against K. pneumoniae ranged from 0.125 to 2.0 μg/ml GaCi. After this in vitro efficacy was established, two topical formulations with GaCi (0.1% [wt/vol] and 0.3% [wt/vol]) were tested in a murine wound model of MDR K. pneumoniae infection. Gross pathology and histopathology revealed K. pneumoniae-infected wounds appeared to close faster with GaCi treatment and were accompanied by reduced inflammation compared to those of untreated controls. Similarly, quantitative indications of infection remediation, such as reduced weight loss and wound area, suggested that treatment improved outcomes compared to those of untreated controls. Bacterial burdens were measured 1 and 3 days following inoculation, and a 0.5 to 1.5 log reduction of CFU was observed. Lastly, upon scanning electron microscopy analysis, GaCi treatment appeared to prevent biofilm formation on dressings compared to those of untreated controls. These results suggest that with more preclinical testing, a topical application of GaCi may be a promising alternative treatment strategy for K. pneumoniae SSTI.

Design, synthesis and evaluation of novel polypharmacological antichlamydial agents

Discovery of new polypharmacological antibacterial agents with multiple modes of actions can be an alternative to combination therapy and also a possibility to slow development of antibiotic resistance. In support to this hypothesis, we synthesized 16 compounds by combining the pharmacophores of Chlamydia trachomatis inhibitors and inhibitors of type III secretion (T3S) in gram-negative bacteria. In this study we have developed salicylidene acylhydrazide sulfonamides (11c &11d) as new antichlamydial agents that also inhibit T3S in Yersinia pseudotuberculosis.

Proton and gallium(III) binding properties of a biologically active salicylidene acylhydrazide

Bacterial biofilm formation causes a range of problems in our society, especially in health care. Salicylidene acylhydrazides (hydrazones) are promising antivirulence drugs targeting secretion systems used during bacterial infection of host cells. When mixed with the gallium ion they become especially potent as bacterial and biofilm growth-suppressing agents, although the mechanisms through which this occurs are not fully understood. At the base of this uncertainty lies the nature of hydrazone-metal interactions. This study addresses this issue by resolving the equilibrium speciation of hydrazone-gallium aqueous solutions. The protonation constants of the target 2-oxo-2-[N-(2,4,6-trihydroxy-benzylidene)-hydrazino]-acetamide (ME0163) hydrazone species and of its 2,4,6-trihydroxybenzaldehyde and oxamic acid hydrazide building blocks were determined by UV-visible spectrophotometry to achieve this goal. These studies show that the hydrazone is an excessively strong complexing agent for gallium and that its antivirulence properties are predominantly ascribed to monomeric 1:1Ga-ME0163 complexes of various Ga hydrolysis and ME0163 protonation states. The chelation of Ga(III) to the hydrazone also increased the stability of the compounds against acid-induced hydrolysis, making this group of compounds very interesting for biological applications where the Fe-antagonist action of both Ga(III) and the hydrazone can be combined for enhanced biological effect.