Biological activity of manganese(i) tricarbonyl complexes on multidrug-resistant Gram-negative bacteria: From functional studies to in vivo activity in Galleria mellonella

Exploration of Rhenium Bisquinoline Tricarbonyl Complexes for their Antibacterial Properties

Metal complexes have emerged as a promising source for novel classes of antibacterial agents to combat the rise of antimicrobial resistance around the world. In the exploration of the transition metal chemical space for novel metalloantibiotics, the rhenium tricarbonyl moiety has been identified as a promising scaffold. Here we have prepared eight novel rhenium bisquinoline tricarbonyl complexes and explored their antibacterial properties. Significant activity against both Gram-positive and Gram-negative bacteria was observed. However, all complexes also showed significant toxicity against human cells, putting into question the prospects of this specific rhenium compound class as metalloantibiotics. To better understand their biological effects, we conduct the first mode of action studies on rhenium bisquinoline complexes and show that they are able to form pores through bacterial membranes. Their straight-forward synthesis and tuneability suggests that further optimisation of this compound class could lead to compounds with enhanced bacterial specificity.

Anti-Malarial and Multi-Bioactive Co (II), Cu (II) and Ni (II) Salen Complexes: Synthesis, Characterization and Computational Studies

Herein, we report the anti-malarial, anti-bacterial and anti-inflammatory activities of the N2O2 donor tetradentate salen type ligand and its CoL, NiL, and CuL metal complexes. The synthesized compounds were characterized by various spectroscopic analytical methods. The in-vitro anti-malarial investigations revealed that the complex CuL exhibited equipotency with quinine drug having IC50 value 0.25 μg/mL. The compound L showed significant inhibition of bacterial spp. viz. E. Coli, P. Aeruginosa, and S. Aureus (MIC=12.5-50 μg/mL), while the compound CoL (MIC=12.5 μg/mL) exhibited potency against gram-positive bacteria. In the in-vitro anti-inflammatory study, the compound CuL displayed moderate activity than other tested compounds. The compound CuL showed the highest anti-malarial docking score with enzyme pLDH at -8.12 Kcal/mol. The DFT study also gives authentication of higher antimalarial activity of CuL due to high dipole moment. None of the potent compounds was found cytotoxic towards vero cell lines.

Antibacterial activity of Au(I), Pt(II), and Ir(III) biotin conjugates prepared by the iClick reaction: influence of the metal coordination sphere on the biological activity

A series of biotin-functionalized transition metal complexes was prepared by iClick reaction from the corresponding azido complexes with a novel alkyne-functionalized biotin derivative ([Au(triazolatoR,R')(PPh3)], [Pt(dpb)(triazolatoR,R')], [Pt(triazolatoR,R')(terpy)]PF6, and [Ir(ppy)(triazolatoR,R')(terpy)]PF6 with dpb = 1,3-di(2-pyridyl)benzene, ppy = 2-phenylpyridine, and terpy = 2,2':6',2''-terpyridine and R = C6H5, R' = biotin). The complexes were compared to reference compounds lacking the biotin moiety. The binding affinity toward avidin and streptavidin was evaluated with the HABA assay as well as isothermal titration calorimetry (ITC). All compounds exhibit the same binding stoichiometry of complex-to-avidin of 4:1, but the ITC results show that the octahedral Ir(III) compound exhibits a higher binding affinity than the square-planar Pt(II) complex. The antibacterial activity of the compounds was evaluated on a series of Gram-negative and Gram-positive bacterial strains. In particular, the neutral Au(I) and Pt(II) complexes showed significant antibacterial activity against Staphylococcus aureus and Enterococcus faecium at very low micromolar concentrations. The cytotoxicity against a range of eukaryotic cell lines was studied and revealed that the octahedral Ir(III) complex was non-toxic, while the square-planar Pt(II) and linear Au(I) complexes displayed non-selective micromolar activity.

Discovery of antibacterial manganese(i) tricarbonyl complexes through combinatorial chemistry

The continuous rise of antimicrobial resistance is a serious threat to human health and already causing hundreds of thousands of deaths each year. While natural products and synthetic organic small molecules have provided the majority of our current antibiotic arsenal, they are falling short in providing new drugs with novel modes of action able to treat multidrug resistant bacteria. Metal complexes have recently shown promising results as antimicrobial agents, but the number of studied compounds is still vanishingly small, making it difficult to identify promising compound classes or elucidate structure-activity relationships. To accelerate the pace of discovery we have applied a combinatorial chemistry approach to the synthesis of metalloantibiotics. Utilizing robust Schiff-base chemistry and combining 7 picolinaldehydes with 10 aniline derivatives, and 6 axial ligands, either imidazole/pyridine-based or solvent, we have prepared a library of 420 novel manganese tricarbonyl complexes. All compounds were evaluated for their antibacterial properties and 10 lead compounds were identified, re-synthesised and fully characterised. All 10 compounds showed high and broad activity against Gram-positive bacteria. The best manganese complex displayed low toxicity against human cells with a therapeutic index of >100. In initial mode of action studies, we show that it targets the bacterial membrane without inducing pore formation or depolarisation. Instead, it releases its carbon monoxide ligands around the membrane and inhibits the bacterial respiratory chain. This work demonstrates that large numbers of metal complexes can be accessed through combinatorial synthesis and evaluated for their antibacterial potential, allowing for the rapid identification of promising metalloantibiotic lead compounds.

Harnessing the dual antimicrobial mode of action with a lipophilic Mn(II) complex using the principle of the Irving-Williams Series to completely eradicate Staphylococcus aurous

The judicious selection of 5,7-dibromo-2-methy-8-quinolinol (BQ) as a Mn(II) ionophore results in the synthesis of Mn(BQ)2(DMSO)2·DMSO (1), a potent metalloantibiotic with a dual antimicrobial mode of action against four different strains of Staphylococcus aurous (SA) bacteria (MIC = 0.625 μg mL-1). Additionally, 1 can overcome ciprofloxacin-resistance in methicillin-resistant SA bacteria.

The manganese carbonyl complex [Mn(CO)3(tqa-κ3N)]Br: A novel antimicrobial agent with the potential to treat avian pathogenic Escherichia coli (APEC) infections

The development of alternatives to antibiotics is essential for the treatment of animal infections and as a measure to reduce the selective pressure on antibiotics that are critical for human medicine. Metal complexes have been highlighted for their antimicrobial activity against several bacterial pathogens. In particular, manganese carbonyl complexes have shown efficacy against multidrug-resistant Gram-negative pathogens, and relatively low cytotoxicity against avian macrophages and in wax moth larval models. They are thus potential candidates for deployment against Avian Pathogenic Escherichia coli (APEC), the aetiological agent of avian colibacillosis, which results in severe animal welfare issues and financial losses worldwide. This study aimed to determine the efficacy of [Mn(CO)3(tqa-κ3N)]Br in Galleria mellonella and chick models of infection against APEC. The results demonstrated in vitro and in vivo antibacterial activity against all antibiotic-resistant APEC test isolates screened in the study.

Galleria mellonella-A Model for the Study of aPDT-Prospects and Drawbacks

Galleria mellonella is a promising in vivo model insect used for microbiological, medical, and pharmacological research. It provides a platform for testing the biocompatibility of various compounds and the kinetics of survival after an infection followed by subsequent treatment, and for the evaluation of various parameters during treatment, including the host-pathogen interaction. There are some similarities in the development of pathologies with mammals. However, a limitation is the lack of adaptive immune response. Antimicrobial photodynamic therapy (aPDT) is an alternative approach for combating microbial infections, including biofilm-associated ones. aPDT is effective against Gram-positive and Gram-negative bacteria, viruses, fungi, and parasites, regardless of whether they are resistant to conventional treatment. The main idea of this comprehensive review was to collect information on the use of G. mellonella in aPDT. It provides a collection of references published in the last 10 years from this area of research, complemented by some practical experiences of the authors of this review. Additionally, the review summarizes in brief information on the G. mellonella model, its advantages and methods used in the processing of material from these larvae, as well as basic knowledge of the principles of aPDT.

Metals to combat antimicrobial resistance

Bacteria, similar to most organisms, have a love-hate relationship with metals: a specific metal may be essential for survival yet toxic in certain forms and concentrations. Metal ions have a long history of antimicrobial activity and have received increasing attention in recent years owing to the rise of antimicrobial resistance. The search for antibacterial agents now encompasses metal ions, nanoparticles and metal complexes with antimicrobial activity ('metalloantibiotics'). Although yet to be advanced to the clinic, metalloantibiotics are a vast and underexplored group of compounds that could lead to a much-needed new class of antibiotics. This Review summarizes recent developments in this growing field, focusing on advances in the development of metalloantibiotics, in particular, those for which the mechanism of action has been investigated. We also provide an overview of alternative uses of metal complexes to combat bacterial infections, including antimicrobial photodynamic therapy and radionuclide diagnosis of bacterial infections.

Clearing an ESKAPE Pathogen in a Model Organism; A Polypyridyl Ruthenium(II) Complex Theranostic that Treats a Resistant Acinetobacter baumannii Infection in Galleria mellonella

In previous studies we have described the therapeutic action of luminescent dinuclear ruthenium(II) complexes based on the tetrapyridylphenazine, tpphz, bridging ligand on pathogenic strains of Escherichia coli and Enterococcus faecalis. Herein, the antimicrobial activity of the complex against pernicious Gram-negative ESKAPE pathogenic strains of Acinetobacter baumannii (AB12, AB16, AB184 and AB210) and Pseudomonas aeruginosa (PA2017, PA_ 007_ IMP and PA_ 004_ CRCN) are reported. Estimated minimum inhibitory concentrations and minimum bactericidal concentrations for the complexes revealed the complex shows potent activity against all A. baumannii strains, in both glucose defined minimal media and standard nutrient rich Mueller-Hinton-II. Although the activity was lower in P. aureginosa, a moderately high potency was observed and retained in carbapenem-resistant strains. Optical microscopy showed that the compound is rapidly internalized by A. baumannii. As previous reports had revealed the complex exhibited no toxicity in Galleria Mellonella up to concentrations of 80 mg/kg, the ability to clear pathogenic infection within this model was explored. The pathogenic concentrations to the larvae for each bacterium were determined to be≥105 for AB184 and≥103 CFU/mL for PA2017. It was found a single dose of the compound totally cleared a pathogenic A. baumannii infection from all treated G. mellonella within 96 h. Uniquely, in these conditions thanks to the imaging properties of the complex the clearance of the bacteria within the hemolymph of G. mellonella could be directly visualized through both optical and transmission electron microscopy.

Metal Complexes as Antifungals? From a Crowd-Sourced Compound Library to the First In Vivo Experiments

There are currently fewer than 10 antifungal drugs in clinical development, but new fungal strains that are resistant to most current antifungals are spreading rapidly across the world. To prevent a second resistance crisis, new classes of antifungal drugs are urgently needed. Metal complexes have proven to be promising candidates for novel antibiotics, but so far, few compounds have been explored for their potential application as antifungal agents. In this work, we report the evaluation of 1039 metal-containing compounds that were screened by the Community for Open Antimicrobial Drug Discovery (CO-ADD). We show that 20.9% of all metal compounds tested have antimicrobial activity against two representative Candida and Cryptococcus strains compared with only 1.1% of the >300,000 purely organic molecules tested through CO-ADD. We identified 90 metal compounds (8.7%) that show antifungal activity while not displaying any cytotoxicity against mammalian cell lines or hemolytic properties at similar concentrations. The structures of 21 metal complexes that display high antifungal activity (MIC ≤1.25 μM) are discussed and evaluated further against a broad panel of yeasts. Most of these have not been previously tested for antifungal activity. Eleven of these metal complexes were tested for toxicity in the Galleria mellonella moth larva model, revealing that only one compound showed signs of toxicity at the highest injected concentration. Lastly, we demonstrated that the organo-Pt(II) cyclooctadiene complex Pt1 significantly reduces fungal load in an in vivo G. mellonella infection model. These findings showcase that the structural and chemical diversity of metal-based compounds can be an invaluable tool in the development of new drugs against infectious diseases.

Coordination complexes constructed from pyrazole-acetamide and pyrazole-quinoxaline: effect of hydrogen bonding on the self-assembly process and antibacterial activity

Two mononuclear coordination complexes of N-(2-aminophenyl)-2-(5-methyl-1H-pyrazol-3-yl)acetamide (L1), namely [Cd(L1)2Cl2] (C1) and [Cu(L1)2(C2H5OH)2](NO3)2 (C2) and one mononuclear complex [Fe(L2)2(H2O)2](NO3)2·2H2O (C3), obtained after in situ oxidation of L1, have been synthesized and characterized spectroscopically. As revealed by single-crystal X-ray diffraction, each coordination sphere made of two heterocycles is completed either by two chloride anions (in C1), two ethanol molecules (in C2) or two water molecules (in C3). The crystal packing analysis of C1, C2 and C3, revealed 1D and 2D supramolecular architectures, respectively, via various hydrogen bonding interactions, which are discussed in detail. Furthermore, evaluation in vitro of the ligands and their metal complexes for their antibacterial activity against Escherichia coli (ATCC 4157), Pseudomonas aeruginosa (ATCC 27853), Staphylococcus aureus (ATCC 25923) and Streptococcus fasciens (ATCC 29212) strains of bacteria, revealed outstanding results compared to chloramphenicol, a well-known antibiotic, with a normalized minimum inhibitory concentration as low as 5 μg mL-1.

Galleria mellonella: The Versatile Host for Drug Discovery, In Vivo Toxicity Testing and Characterising Host-Pathogen Interactions

Larvae of the greater wax moth, Galleria mellonella, are a convenient in vivo model for assessing the activity and toxicity of antimicrobial agents and for studying the immune response to pathogens and provide results similar to those from mammals. G. mellonella larvae are now widely used in academia and industry and their use can assist in the identification and evaluation of novel antimicrobial agents. Galleria larvae are inexpensive to purchase and house, easy to inoculate, generate results within 24–48 h and their use is not restricted by legal or ethical considerations. This review will highlight how Galleria larvae can be used to assess the efficacy of novel antimicrobial therapies (photodynamic therapy, phage therapy, metal-based drugs, triazole-amino acid hybrids) and for determining the in vivo toxicity of compounds (e.g., food preservatives, ionic liquids) and/or solvents (polysorbate 80). In addition, the disease development processes are associated with a variety of pathogens (e.g., Staphylococcus aureus, Listeria monocytogenes, Aspergillus fumigatus, Madurella mycotomatis) in mammals are also present in Galleria larvae thus providing a simple in vivo model for characterising disease progression. The use of Galleria larvae offers many advantages and can lead to an acceleration in the development of novel antimicrobials and may be a prerequisite to mammalian testing.

Establishing an invertebrate Galleria mellonella greater wax moth larval model of Neisseria gonorrhoeae infection

Neisseria gonorrhoeae (gonococcus) causes the human sexually transmitted disease gonorrhea. Studying gonococcal pathogenesis and developing new vaccines and therapies to combat the increasing prevalence of multi-antibiotic resistant bacteria has made use of many ex vivo models based on human cells and tissues, and in vivo vertebrate models, for example, rodent, pig and human. The focus of the current study was to examine the utility of the invertebrate greater wax moth Galleria mellonella as an in vivo model of gonococcal infection. We observed that a threshold of ~106 - 107 gonococci/larva was required to kill >50% of larvae (P < 0.05), and increased toxicity correlated with reduced health index scores and pronounced histopathological changes such as increases in the total lesion grade, melanized nodules, hemocyte reaction, and multifocal adipose body degeneration. Larval death was independent of the expression of pilus or Opa protein or LOS sialylation within a single gonococcal species studied, but the model could demonstrate relative toxicity of different isolates. N. meningitidis, N. lacatamica and gonococci all killed larvae equally, but were significantly less toxic (P > 0.05) than Pseudomonas aeruginosa. Larvae primed with nontoxic doses of gonococci were more susceptible to subsequent challenge with homologous and heterologous bacteria, and larval survival was significantly reduced (P < 0.05) in infected larvae after depletion of their hemocytes with clodronate-liposomes. The model was used to test the anti-gonococcal properties of antibiotics and novel antimicrobials. Ceftriaxone (P < 0.05) protected larvae from infection with different gonococcal isolates, but not azithromycin or monocaprin or ligand-coated silver nanoclusters (P > 0.05).

Galleria mellonella as a Good Model to Study Acinetobacter baumannii Pathogenesis

The invertebrate model, Galleria mellonella, has been widely used to study host-pathogen interactions due to its cheapness, ease of handling, and similar mammalian innate immune system. G. mellonella larvae have been proven to be useful and a reliable model for analyzing pathogenesis mechanisms of multidrug resistant Acinetobacter baumannii, an opportunistic pathogen difficult to kill. This review describes the detailed experimental design of G. mellonella/A. baumannii models, and provides a comprehensive comparison of various virulence factors and therapy strategies using the G. mellonella host. These investigations highlight the importance of this host-pathogen model for in vivo pathogen virulence studies. On the long term, further development of the G. mellonella/A. baumannii model will offer promising insights for clinical treatments of A. baumannii infection.

Brief survey on organometalated antibacterial drugs and metal-based materials with antibacterial activity

Rising bacterial antibiotic resistance is a global threat. To deal with it, new antibacterial agents and antiseptic materials need to be developed. One alternative in this quest is the organometallic derivatization of well-established antibacterial drugs and also the fabrication of advanced metal-based materials having antibacterial properties. Metal-based agents and materials often show new modes of antimicrobial action which enable them to overcome drug resistance in pathogenic bacterial strains. This review summarizes recent (2017-2020) progress in the field of organometallic-derived antibacterial drugs and metal-based materials having antibacterial activity. Specifically, it covers organometallic derivatives of antibacterial drugs including β-lactams, ciprofloxacin, isoniazid, trimethoprim, sulfadoxine, sulfamethoxazole, and ethambutol as well as non-antibacterial drugs like metformin, phenformin and aspirin. Recent advances and reported clinical trials in the use of metal-based nanomaterials as antibiofouling coatings on medical devices, as photocatalytic agents in indoor air pollutant control, and also as photodynamic/photothermal antimicrobial agents are also summarized.

Development of photoactivable phenanthroline-based manganese(I) CO-Releasing molecules (PhotoCORMs) active against ESKAPE bacteria and bacterial biofilms

The synthesis and biological evaluation of a series of phenanthroline-based visible-light-activated manganese(I) carbon-monoxide-releasing molecules (PhotoCORMs) against ESKAPE bacteria and bacterial biofilms is reported. Four carbonyl compounds of general formula fac-[Mn(N^∧N)(CO)₃(L)] have been synthesized and characterized. Despite being thermally stable in the absence of light, these PhotoCORMs readily release CO upon blue (435-450 nm) LED light irradiation as confirmed by spectrophotometric CO releasing experiments (Mb Assay). The antibacterial activity of the four PhotoCORMs has been investigated against a panel of ESKAPE bacteria. The compounds 1-3 were found to be effective antibacterials at low concentrations against multidrug-resistant Klebsiella pneumoniae and Acinetobacter baumannii when photoactivated with blue-light. In addition, the PhotoCORMs 1-2 were found to inhibit the formation of Klebsiella pneumoniae and Acinetobacter baumannii bacterial biofilms at low concentrations (MIC = 4-8 μg/mL), turning out to be promising candidates to combat antimicrobial resistance. The antibacterial and biofilm inhibitory effect of the PhotoCORMs is plausibly due to the release of CO as well as the formation of phenanthroline photo-by-products as revealed by spectroscopy and microbiology experiments.

Role of Carbon Monoxide in Host-Gut Microbiome Communication

Nature is full of examples of symbiotic relationships. The critical symbiotic relation between host and mutualistic bacteria is attracting increasing attention to the degree that the gut microbiome is proposed by some as a new organ system. The microbiome exerts its systemic effect through a diverse range of metabolites, which include gaseous molecules such as H₂, CO₂, NH₃, CH₄, NO, H₂S, and CO. In turn, the human host can influence the microbiome through these gaseous molecules as well in a reciprocal manner. Among these gaseous molecules, NO, H₂S, and CO occupy a special place because of their widely known physiological functions in the host and their overlap and similarity in both targets and functions. The roles that NO and H₂S play have been extensively examined by others. Herein, the roles of CO in host-gut microbiome communication are examined through a discussion of (1) host production and function of CO, (2) available CO donors as research tools, (3) CO production from diet and bacterial sources, (4) effect of CO on bacteria including CO sensing, and (5) gut microbiome production of CO. There is a large amount of literature suggesting the "messenger" role of CO in host-gut microbiome communication. However, much more work is needed to begin achieving a systematic understanding of this issue.

UtilisingGalleria mellonella larvae for studying in vivo activity of conventional and novel antimicrobial agents

The immune response of insects displays many structural and functional similarities to the innate immune response of mammals. As a result of these conserved features, insects may be used for evaluating microbial virulence or for testing the in vivo efficacy and toxicity of antimicrobial compounds and results show strong similarities to those from mammals. Galleria mellonella larvae are widely used in this capacity and have the advantage of being easy to use, inexpensive to purchase and house, and being free from the ethical and legal restrictions that relate to the use of mammals in these tests. Galleria mellonella larvae may be used to assess the in vivo toxicity and efficacy of novel antimicrobial compounds. A wide range of antibacterial and antifungal therapies have been evaluated in G. mellonella larvae and results have informed subsequent experiments in mammals. While insect larvae are a convenient and reproducible model to use, care must be taken in their use to ensure accuracy of results. The objective of this review is to provide a comprehensive account of the use of G. mellonella larvae for assessing the in vivo toxicity and efficacy of a wide range of antibacterial and antifungal agents.

Design, synthesis and in vivo evaluation of 3-arylcoumarin derivatives of rhenium(I) tricarbonyl complexes as potent antibacterial agents against methicillin-resistant Staphylococcus aureus (MRSA)

We have prepared a series of ten 3-arylcoumarin molecules, their respective fac-[Re(CO)₃(bpy)L]⁺ and fac-[Re(CO)₃(L⁀L)Br] complexes and tested all compounds for their antimicrobial efficacy. Whereas the 3-arylcoumarin ligands are virtually inactive against the human-associated pathogens with minimum inhibitory concentrations (MICs) > 150 μM, when coordinated to the fac-[Re(CO)₃]⁺ core, most of the resulting complexes showed remarkable antibacterial potency. Several rhenium complexes exhibit activity in nanomolar concentrations against Gram-positive pathogens such as Staphylococcus aureus strains, including methicillin-resistant S. aureus (MRSA) and Enterococcus faecium. The molecules do not affect bacterial cell membrane potential, but some of the most potent complexes strongly interact with DNA, indicating it as a possible target for their mode of action. In vivo studies in the zebrafish model showed that the complexes with anti-staphylococcal/MRSA activity were non-toxic to the organism even at much higher doses of the corresponding MICs. In the zebrafish-MRSA infection model, the complexes increased the survival rate of infected fish up to 100% and markedly reduced bacterial burden. Moreover, all rescued fish developed normally following the treatments with the metallic compounds.

Photo-activated CO-release in the amino tungsten Fischer carbene complex, [(CO)5WC(NC4H8)Me], picosecond time resolved infrared spectroscopy, time-dependent density functional theory, and an antimicrobial study

Picosecond time-resolved infrared spectroscopy was used to probe the photo-induced early state dynamics preceding CO loss in the Fischer carbene complex, [(CO)₅WC(NC₄H₈)CH₃]. Time-dependent density functional theory calculations were employed to help in understanding the photochemical and photophysical processes leading to CO-loss. Electrochemical initiated CO release was quantified using gas chromatography. The potential of [(CO)₅WC(NC₄H₈)CH₃], as an antimicrobial agent under irradiation conditions was studied using a Staphylococcus aureus strain.

Recent Studies on the Antimicrobial Activity of Transition Metal Complexes of Groups 6–12

Antimicrobial resistance is an increasingly serious threat to global public health that requires innovative solutions to counteract new resistance mechanisms emerging and spreading globally in infectious pathogens. Classic organic antibiotics are rapidly exhausting the structural variations available for an effective antimicrobial drug and new compounds emerging from the industrial pharmaceutical pipeline will likely have a short-term and limited impact before the pathogens can adapt. Inorganic and organometallic complexes offer the opportunity to discover and develop new active antimicrobial agents by exploiting their wide range of three-dimensional geometries and virtually infinite design possibilities that can affect their substitution kinetics, charge, lipophilicity, biological targets and modes of action. This review describes recent studies on the antimicrobial activity of transition metal complexes of groups 6–12. It focuses on the effectiveness of the metal complexes in relation to the rich structural chemical variations of the same. The aim is to provide a short vade mecum for the readers interested in the subject that can complement other reviews.

Manganese complex [Mn(CO)3(tpa-κ3N)]Br increases antibiotic sensitivity in multidrug resistant Streptococcus pneumoniae

OBJECTIVES

The emergence of multidrug-resistance (MDR) in Streptococcus pneumoniae clones and non-vaccine serotypes necessitate the development of novel treatment strategies. This work aimed to determine the efficacy of the Mn complex [Mn(CO)³(tpa-κ³N)]Br against clinically important MDR strains of S. pneumoniae.

METHODS

Twenty MDR clinicalS. pneumoniae strains were included in this study. Minimum inhibitory concentrations (MICs) of [Mn(CO)₃(tpa-κ³N)]Br were determined via broth microdilution alone and in combination with other antimicrobial agents using checkerboard assays and/or disc diffusion tests. In vitro efficacy was assessed by time-kill assays while in vivo efficacy was tested using the insect model Galleria mellonella.

RESULTS

[Mn(CO)₃(tpa-κ³N)]Br showed moderate in vitro efficacy against S. pneumoniae coupled with bactericidal activity. Checkerboard and disc diffusion assays showed synergy between [Mn(CO)₃(tpa-κ³N)]Br and tetracycline, and the combination of both agents caused rapid kill-kinetics and reduced the MIC below the susceptibility breakpoint of 1 mg/L even for tetracycline-resistant strains of S. pneumoniae. Similar results were observed for the erythromycin- and the co-trimoxazole-Mn complex combination. In the G. mellonella infection model, mortality and morbidity rates at 96 h were significantly lower in larvae treated with [Mn(CO)₃(tpa-κ³N)]Br than phosphate buffered saline, while treatment with the tetracycline-Mn complex combination was superior to monotherapy, resulting in significantly lower mortality and morbidity rates (p < 0.049).

CONCLUSIONS

We show that [Mn(CO)₃(tpa-κ³N)]Br has in vitro and in vivo antibacterial activity against clinically relevant strains of S. pneumoniae and has the potential to be used in combination with currently available antibiotics to increase their effectiveness against MDR S. pneumoniae.

Antibacterial activity of Mn(I) and Re(I) tricarbonyl complexes conjugated to a bile acid carrier molecule

A bifunctional cholic acid-bis(2-pyridylmethyl)amine (bpa) ligand featuring an amide linker was coordinated to a manganese(i) or rhenium(i) tricarbonyl moiety to give [M(bpacholamide)(CO)3] with M = Mn, Re in good yield and very high purity. Strong antibacterial activity was observed against four strains of methicillin-susceptible (MSSA) and methicillin-resistant (MRSA) Staphylococcus aureus, with minimum inhibitory concentrations (MICs) in the range of 2-3.5 μM. No difference in response was observed for the MSSA vs. MRSA strains. Activity was also independent of the nature of the metal center, as the Mn and Re complexes showed essentially identical MIC values. In contrast to some other metal carbonyl complexes, the activity seems to be unrelated to the release of carbon monoxide, as photoactivation of the Mn complex reduced the potency by a factor of 2-8. Both metal complexes were non-toxic in Galleria mellonella larvae at concentrations of up to 100× the MIC value. In vivo testing in Galleria larvae infected with MRSA/MSSA demonstrated a significant increase in overall survival rates from 46% in the control to 88% in the group treated with the metal complexes. ICP-MS analysis showed that the Mn and Re cholamide complexes are efficiently internalized by E. coli cells and do not interfere with membrane integrity, as evident from a lack of release of intracellular ATP. An increased sensitivity was observed in acrB, acrD, and mdt mutants that are defective in multidrug exporters, indicating that the compounds have an intracellular mechanism of action. Furthermore, E. coli mntP mutants defective in the gene encoding an Mn exporter were more sensitive than the wildtype, while inactivation of the regulator that controls expression of the Mn uptake proteins MntP and MntH slightly increased sensitivity to the compound. Single knockout mutants defective in genes linked to bile salt and oxidative stress response (dinF, yiaH, sodA, katE, and soxS) did not show increased sensitivity relative to the wild type. Overall, neither the cholic acid moiety nor the metal-carbonyl fragment alone appear to be responsible for the biological activity observed and thus the search for the primary intracellular target continues.