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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 hours. 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.

When the ‘CO-releasing molecule-3’, CORM-3 (Ru(CO)3Cl(glycinate)), is dissolved in water it forms a range of ruthenium complexes. These are taken up by cells and bind to intracellular ligands, notably thiols such as cysteine and glutathione, where the Ru(II) reaches high intracellular concentrations. Here, we show that the Ru(II) ion also binds to DNA, at exposed guanosine N7 positions. It therefore has a similar cellular target to the anticancer drug cisplatin, but not identical, because Ru(II) shows no evidence of forming intramolecular crossbridges in the DNA. The reaction is slow, and with excess Ru, intermolecular DNA crossbridges are formed. The addition of CORM-3 to human colorectal cancer cells leads to strand breaks in the DNA, as assessed by the alkaline comet assay. DNA damage is inhibited by growth media containing amino acids, which bind to extracellular Ru and prevent its entry into cells. We conclude that the cytotoxicity of Ru(II) is different from that of platinum, making it a promising development target for cancer therapeutics.

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.

Carbon monoxide (CO)-releasing molecules (CORMs) are used to deliver CO, a biological ‘gasotransmitter’, in biological chemistry and biomedicine. CORMs kill bacteria in culture and in animal models, but are reportedly benign towards mammalian cells. CORM-2 (tricarbonyldichlororuthenium(II) dimer, Ru2Cl4(CO)6), the first widely used and commercially available CORM, displays numerous pharmacological, biochemical and microbiological activities, generally attributed to CO release. Here, we investigate the basis of its potent antibacterial activity against Escherichia coli and demonstrate, using three globin CO sensors, that CORM-2 releases negligible CO (&lt, 0.1 mol CO per mol CORM-2). A strong negative correlation between viability and cellular ruthenium accumulation implies that ruthenium toxicity underlies biocidal activity. Exogenous amino acids and thiols (especially cysteine, glutathione and N-acetyl cysteine) protected bacteria against inhibition of growth by CORM-2. Bacteria treated with 30 μM CORM-2, with added cysteine and histidine, exhibited no significant loss of viability, but were killed in the absence of these amino acids. Their prevention of toxicity correlates with their CORM-2-binding affinities (Cys, Kd 3 μM, His, Kd 130 μM) as determined by 1H-NMR. Glutathione is proposed to be an important intracellular target of CORM-2, with CORM-2 having a much higher affinity for reduced glutathione (GSH) than oxidised glutathione (GSSG) (GSH, Kd 2 μM, GSSG, Kd 25,000 μM). The toxicity of low, but potent, levels (15 μM) of CORM-2 was accompanied by cell lysis, as judged by the release of cytoplasmic ATP pools. The biological effects of CORM-2 and related CORMs, and the design of biological experiments, must be re-examined in the light of these data.

Six luminescent, mononuclear ruthenium(ii) complexes based on the tetrapyridophenazine (tpphz) and dipyridophenazine (dppz) ligands are reported. The therapeutic activities of the complexes against Gram-negative bacteria (E. coli, A. baumannii, P. aeruginosa) and Gram-positive bacteria (E. faecalis and S. aureus) including pathogenic multi- and pan-drug resistant strains were assessed. Estimated minimum inhibitory and bactericidal concentrations show the activity of the lead compound is comparable to ampicillin and oxacillin in therapeutically sensitive strains and this activity was retained in resistant strains. Unlike related dinuclear analogues the lead compound does not damage bacterial membranes but is still rapidly taken up by both Gram-positive and Gram-negative bacteria in a glucose independent manner. Direct imaging of the complexes through super-resolution nanoscopy and transmission electron microscopy reveals that once internalized the complexes' intracellular target for both Gram-negative and Gram-positive strains is bacterial DNA. Model toxicity screens showed the compound is non-toxic to Galleria mellonella even at exposure concentrations that are orders of magnitude higher than the bacterial MIC., A mononuclear ruthenium(ii) complex based of the tpphz ligand is shown to be a broad-band antimicrobial theranostic active against a range of AMR pathogens.

The cis-α isomer of [Ru(bb7)(dppz)]2+ {dppz = dipyrido[3,2-a:2′,3′-c]phenazine; bb7 = bis[4(4'-methyl-2,2'-bipyridyl)]-1,7-alkane} has been synthesised. The minimum inhibitory concentrations and the minimum bactericidal concentrations of [Ru(bb7)(dppz)]2+ and its parent complex [Ru(phen)2(dppz)]2+ (phen = 1,10-phenanthroline) were determined against a range of bacteria. The results showed that both ruthenium complexes exhibited good activity against Gram-positive bacteria, but [Ru(bb7)(dppz)]2+ showed at least eight-times better activity across the Gram-negative bacteria than [Ru(phen)2(dppz)]2+. Luminescence assays demonstrated that [Ru(bb7)(dppz)]2+ accumulated in a Gram-negative bacterium to the same degree as in a Gram-positive species, while assays with liposomes showed that [Ru(bb7)(dppz)]2+ interacted more strongly with membranes compared to the parent [Ru(phen)2(dppz)]2+ complex. The DNA binding affinity for [Ru(bb7)(dppz)]2+ was determined to be 6.7 × 106 M-1. Although more toxic to eukaryotic cells than [Ru(phen)2(dppz)]2+, [Ru(bb7)(dppz)]2+ exhibited greater activity against bacteria than eukaryotic cells.

Linear and non-linear tetranuclear ruthenium(ii) complexes containing the bridging ligand bis[4(4'-methyl-2,2'-bipyridyl)]-1,7-heptane have been synthesised and their biological properties examined. The minimum inhibitory concentrations (MIC) and the minimum bactericidal concentrations (MBC) of the ruthenium(ii) complexes were determined against six strains of bacteria: Gram-positive Staphylococcus aureus (S. aureus) and methicillin-resistant S. aureus (MRSA); and the Gram-negative Escherichia coli (E. coli) strains MG1655, APEC, UPEC and Pseudomonas aeruginosa (P. aeruginosa). The results showed that both tetranuclear complexes had significant antimicrobial activity, with the non-linear (branched) species (Rubb7-TNL) having slightly higher activity than the corresponding linear analogue (Rubb7-TL). The corresponding toxicity against three eukaryotic cell lines - BHK (baby hamster kidney), Caco-2 (heterogeneous human epithelial colorectal adenocarcinoma) and Hep-G2 (liver carcinoma) - have also been determined. Interestingly, both Rubb7-TNL and Rubb7-TL were as toxic to the eukaryotic cells as they were to the bacteria, a rarity for kinetically-inert cationic polypyridylruthenium(ii) complexes, and exhibited lower IC50 values than cisplatin over 24-, 48- or 72-hour incubation times. Fluorescence spectroscopy was used to study the binding of the ruthenium complexes with human serum albumin (HSA). Rubb7-TNL and Rubb7-TL exhibited strong HSA binding, with equilibrium binding constants in the order of 107 M-1. Confocal microscopy was used to examine the cellular localisation of Rubb7-TNL in BHK cells. The results indicated that the ruthenium complex localised in the nucleolus. Significant accumulation was also observed in the cytoplasm, but not in the mitochondria. Taken together, the results of this study suggest that Rubb7-TNL is an unlikely candidate as an antimicrobial agent, but may have potential as an anticancer drug.

Three new manganese(I) tricarbonyl complexes [Mn(bpqa-κ3N)(CO)3]Br, [Mn(bqpa-κ3N)(CO)3]Br, and [Mn(CO)3(tqa-κ3N)]Br as well as the previously described compound [Mn(CO)3(tpa-κ3N)]Br with bpqa = bis(2-pyridinylmethyl)(2-quinolinylmethyl)amine, bqpa = bis(2-quinolinylmethyl)(2-pyridinylmethyl)amine, tqa = tris(2-quinolinylmethyl)amine, and tpa = tris(2-pyridinylmethyl)amine were examined for their antibacterial activities on 14 different multidrug-resistant clinical isolates of Acinetobacter baumannii and Pseudomonas aeruginosa, in recognition of the current antimicrobial resistance (AMR) concerns with these pathogens. Minimal inhibitory concentrations (MIC) of the most potent tqa compound were in the mid-micromolar range and generally lower than that of the free ligand. Activity against both bacterial species increased with the number of quinolinylmethyl groups and lipophilicity in the order of tpa < bpqa < bqpa ≈ tqa, consistent with measured increases in release of ATP, a uniquely cytoplasmic biomolecule and induced permeability to exogenous fluorescent intercalating compounds. [Mn(CO)3(tqa-κ3N)]Br was also evaluated in the Galleria mellonella model of infection, and displayed a lack of host toxicity combined with effective bacterial clearance.

Medicinal leads that are also compatible with imaging technologies are attractive, as they facilitate the development of therapeutics through direct mechanistic observations at the molecular level. In this context, the uptake and antimicrobial activities of several luminescent dinuclear RuII complexes against E. coli were assessed and compared to results obtained for another ESKAPE pathogen, the Gram-positive major opportunistic pathogen Enterococcus faecalis, V583. The most promising lead displays potent activity, particularly against the Gram-negative bacteria, and potency is retained in the uropathogenic multidrug resistant EC958 ST131 strain. Exploiting the inherent luminescent properties of this complex, super-resolution STED nanoscopy was used to image its initial localization at/in cellular membranes and its subsequent transfer to the cell poles. Membrane damage assays confirm that the complex disrupts the bacterial membrane structure before internalization. Mammalian cell culture and animal model studies indicate that the complex is not toxic to eukaryotes, even at concentrations that are several orders of magnitude higher than its minimum inhibitory concentration (MIC). Taken together, these results have identified a lead molecular architecture for hard-to-treat, multiresistant, Gram-negative bacteria, which displays activities that are already comparable to optimized natural product-based leads.

Carbon monoxide (CO)-releasing molecules (CORMs), mostly metal carbonyl compounds, are extensively used as experimental tools to deliver CO, a biological ‘gasotransmitter’, in mammalian systems. CORMs are also explored as potential novel antimicrobial drugs, effectively and rapidly killing bacteria in vitro and in animal models, but are reportedly benign towards mammalian cells. Ru-carbonyl CORMs, exemplified by CORM-3 (Ru(CO)3Cl(glycinate)), exhibit the most potent antimicrobial effects against Escherichia coli. We demonstrate that CORM-3 releases little CO in buffers and cell culture media and that the active antimicrobial agent is Ru(II), which binds tightly to thiols. Thus, thiols and amino acids in complex growth media – such as histidine, methionine and oxidised glutathione, but most pertinently cysteine and reduced glutathione (GSH) – protect both bacterial and mammalian cells against CORM-3 by binding and sequestering Ru(II). No other amino acids exert significant protective effects. NMR reveals that CORM-3 binds cysteine and GSH in a 1:1 stoichiometry with dissociation constants, Kd, of about 5 μM, while histidine, GSSG and methionine are bound less tightly, with Kd values ranging between 800 and 9000 μM. There is a direct positive correlation between protection and amino acid affinity for CORM-3. Intracellular targets of CORM-3 in both bacterial and mammalian cells are therefore expected to include GSH, free Cys, His and Met residues and any molecules that contain these surface-exposed amino acids. These results necessitate a major reappraisal of the biological effects of CORM-3 and related CORMs., Graphical abstract fx1, Highlights • Carbon monoxide-releasing molecules (CORMs) are used for experimental CO delivery. • CORM-3 is a potent antimicrobial, but is reportedly beneficial to mammalian cells. • We demonstrate CORM-3 releases little CO in buffers and cell culture media. • Redox-active Ru2+ is the biological agent, binding tightly to metabolites e.g. thiol. • These results necessitate a major reappraisal of the biological effects of CORMs.

A gasotransmitter is defined as a small, generally reactive, gaseous molecule that, in solution, is generated endogenously in an organism and exerts important signalling roles. It is noteworthy that these molecules are also toxic and antimicrobial. We ask: is this definition of a gasotransmitter appropriate in the cases of nitric oxide, carbon monoxide and hydrogen sulfide (H2S) in microbes? Recent advances show that, not only do bacteria synthesise each of these gases, but the molecules also have important signalling or messenger roles in addition to their toxic effects. However, strict application of the criteria proposed for a gasotransmitter leads us to conclude that the term ‘small molecule signalling agent’, as proposed by Fukuto and others, is preferable terminology.

A series of mononuclear ruthenium(II) complexes containing the tetradentate ligand bis[4(4’-methyl-2,2’-\ud bipyridyl)]-1,7-heptane have been synthesised and their biological properties examined. In the synthesis\ud of the [Ru(phen’)(bb7)]2+ complexes (where phen’ = 1,10-phenanthroline and its 5-nitro-, 4,7-dimethyland\ud 3,4,7,8-tetramethyl- derivatives), both the symmetric cis-α and non-symmetric cis-β isomers\ud were formed. However, upon standing for a number of days (or more quickly under harsh conditions) the\ud cis-β isomer converted to the more thermodynamically stable cis-α isomer. The minimum inhibitory concentrations\ud (MIC) and the minimum bactericidal concentrations (MBC) of the ruthenium(II) complexes\ud were determined against six strains of bacteria: Gram-positive Staphylococcus aureus (S. aureus) and\ud methicillin-resistant S. aureus (MRSA); and the Gram-negative Escherichia coli (E. coli) strains MG1655,\ud APEC, UPEC and Pseudomonas aeruginosa (P. aeruginosa). The results showed that the [Ru(5-NO2phen)-\ud (bb7)]2+ complex had little or no activity against any of the bacterial strains. By contrast, for the other cisα-[Ru(phen’)(bb7)]2+\ud complexes, the antimicrobial activity increased with the degree of methylation. In\ud particular, the cis-α-[Ru(Me4phen)(bb7)]2+ complex showed excellent and uniform MIC activity against all\ud bacteria. By contrast, the MBC values for the cis-α-[Ru(Me4phen)(bb7)]2+ complex varied considerably\ud across the bacteria and even within S. aureus and E. coli strains. In order to gain an understanding of the\ud relative antimicrobial activities, the DNA-binding affinity, cellular accumulation and water–octanol partition\ud coefficients (log P) of the ruthenium complexes were determined. Interestingly, all the [Ru(phen’)-\ud (bb7)]2+ complexes exhibited stronger DNA binding affinity (Ka ≈ 1 × 107 M−1\ud ) than the well-known DNAintercalating\ud complex [Ru(phen)2(dppz)]2+ (where dppz = dipyrido[3,2-a:2’,3’-c]phenazine).

Ruthenium is seldom mentioned in microbiology texts, due to the fact that this metal has no known, essential roles in biological systems, nor is it generally considered toxic. Since the fortuitous discovery of cisplatin, first as an antimicrobial agent and then later employed widely as an anticancer agent, complexes of other platinum group metals, such as ruthenium, have attracted interest for their medicinal properties. Here, we review at length how ruthenium complexes have been investigated as potential antimicrobial, antiparasitic and chemotherapeutic agents, in addition to their long and well-established roles as biological stains and inhibitors of calcium channels. Ruthenium complexes are also employed in a surprising number of biotechnological roles. It is in the employment of ruthenium complexes as antimicrobial agents and alternatives or adjuvants to more traditional antibiotics, that we expect to see the most striking developments in the future. Such novel contributions from organometallic chemistry are undoubtedly sorely needed to address the antimicrobial resistance crisis and the slow appearance on the market of new antibiotics.

© 2017 Elsevier Ltd. Ruthenium is seldom mentioned in microbiology texts, due to the fact that this metal has no known, essential roles in biological systems, nor is it generally considered toxic. Since the fortuitous discovery of cisplatin, first as an antimicrobial agent and then later employed widely as an anticancer agent, complexes of other platinum group metals, such as ruthenium, have attracted interest for their medicinal properties. Here, we review at length how ruthenium complexes have been investigated as potential antimicrobial, antiparasitic and chemotherapeutic agents, in addition to their long and well-established roles as biological stains and inhibitors of calcium channels. Ruthenium complexes are also employed in a surprising number of biotechnological roles. It is in the employment of ruthenium complexes as antimicrobial agents and alternatives or adjuvants to more traditional antibiotics, that we expect to see the most striking developments in the future. Such novel contributions from organometallic chemistry are undoubtedly sorely needed to address the antimicrobial resistance crisis and the slow appearance on the market of new antibiotics.