Antibacterial effect of the scandium and indium complexes of enterochelin on Escherichia coli

Antibacterial, remineralising and matrix metalloproteinase inhibiting scandium-doped phosphate glasses for treatment of dental caries

OBJECTIVES

Antibiotic resistance is increasingly a growing global threat. This study aimed to investigate the potential use of newly developed scandium-doped phosphate-based glasses (Sc-PBGs) as an antibacterial and anticariogenic agent through controlled release of Sc³⁺ ions.

METHODS

Sc-PBGs with various calcium and sodium oxide contents were produced and characterised using thermal and spectroscopic analysis. Degradation behaviour, ion release, antibacterial action against Streptococcus mutans, anti-matrix metalloproteinase-2 (MMP-2) activity, remineralisation potential and in vivo biocompatibility were also investigated.

RESULTS

The developed glass system showed linear Sc³⁺ ions release over time. The released Sc³⁺ shows statistically significant inhibition of S. mutans biofilm (1.2 log₁₀ CFU reduction at 6 h) and matrix metalloproteinase-2 (MMP-2) activity, compared with Sc-free glass and positive control. When Sc-PBGs were mounted alongside enamel sections, subjected to acidic challenges, alternating hyper- and hypomineralisation layers consistent with periods of re- and demineralisation were observed demonstrating their potential remineralising action. Furthermore, Sc-PBGs produced a non-toxic response when implanted subcutaneously for 2 weeks in Sprague Dawley rats.

SIGNIFICANCE

Since Sc³⁺ ions might act on various enzymes essential to the biological mechanisms underlying caries, Sc-PBGs could be a promising therapeutic agent against cariogenic bacteria.

Identification of an iridium(III) complex with anti-bacterial and anti-cancer activity

Group 9 transition metal complexes have been widely explored as therapeutic agents due to their unique geometry, their propensity to undergo ligand exchanges with biomolecules and their diverse steric and electronic properties. These metal complexes can offer distinct modes of action in living organisms compared to carbon-based molecules. In this study, we investigated the antimicrobial and anti-proliferative abilities of a series of cyclometallated iridium(III) complexes. The iridium(III) complex 1 inhibited the growth of S. aureus with MIC and MBC values of 3.60 and 7.19 μM, respectively, indicating its potent bactericidal activity. Moreover, complex 1 also exhibited cytotoxicity against a number of cancer cell lines, with particular potency against ovarian, cervical and melanoma cells. This cyclometallated iridium(III) complex is the first example of a substitutionally-inert, Group 9 organometallic compound utilized as a direct and selective inhibitor of S. aureus.

Antimicrobial activity of metals: mechanisms, molecular targets and applications. Nat Rev Microbiol. 2013;11:371-384. [PMID: 23669886 DOI: 10.1038/nrmicro3028]

Metals have been used as antimicrobial agents since antiquity, but throughout most of history their modes of action have remained unclear. Recent studies indicate that different metals cause discrete and distinct types of injuries to microbial cells as a result of oxidative stress, protein dysfunction or membrane damage. Here, we describe the chemical and toxicological principles that underlie the antimicrobial activity of metals and discuss the preferences of metal atoms for specific microbial targets. Interdisciplinary research is advancing not only our understanding of metal toxicity but also the design of metal-based compounds for use as antimicrobial agents and alternatives to antibiotics.

Undecaprenyl pyrophosphate involvement in susceptibility of Bacillus subtilis to rare earth elements

The rare earth element scandium has weak antibacterial potency. We identified a mutation responsible for a scandium-resistant phenotype in Bacillus subtilis. This mutation was found within the uppS gene, which encodes undecaprenyl pyrophosphate synthase, and designated uppS86 (for the Thr-to-Ile amino acid substitution at residue 86 of undecaprenyl pyrophosphate synthase). The uppS86 mutation also gave rise to increased resistance to bacitracin, which prevents cell wall synthesis by inhibiting the dephosphorylation of undecaprenyl pyrophosphate, in addition to enhanced amylase production. Conversely, overexpression of the wild-type uppS gene resulted in increased susceptibilities to both scandium and bacitracin. Moreover, the mutant lacking undecaprenyl pyrophosphate phosphatase (BcrC) showed increased susceptibility to all rare earth elements tested. These results suggest that the accumulation of undecaprenyl pyrophosphate renders cells more susceptible to rare earth elements. The availability of undecaprenyl pyrophosphate may be an important determinant for susceptibility to rare earth elements, such as scandium.

Exploiting bacterial iron acquisition: siderophore conjugates

Siderophores are chelators synthesized by bacteria and fungi to sequester iron, which is essential for virulence and pathogenicity. Since the process involves active transport, which is highly regulated, remarkably efficient and often microbially selective, it has been exploited as a Trojan Horse method for development of microbe-selective antibiotics. Siderophores also have significant potential for the development of imaging contrast agents and diagnostics for pathogen-selective detection. These promising results demonstrate the versatility of natural and synthetic microbial iron chelators and their potential therapeutic applications.

The potential of desferrioxamine-gallium as an anti-Pseudomonas therapeutic agent

The opportunistic pathogen Pseudomonas aeruginosa causes infections that are difficult to treat by antibiotic therapy. This bacterium can cause biofilm infections where it shows tolerance to antibiotics. Here we report the novel use of a metallo-complex, desferrioxamine-gallium (DFO-Ga) that targets P. aeruginosa iron metabolism. This complex kills free-living bacteria and blocks biofilm formation. A combination of DFO-Ga and the anti-Pseudomonas antibiotic gentamicin caused massive killing of P. aeruginosa cells in mature biofilms. In a P. aeruginosa rabbit corneal infection, topical administration of DFO-Ga together with gentamicin decreased both infiltrate and final scar size by about 50% compared to topical application of gentamicin alone. The use of DFO-Ga as a Trojan horse delivery system that interferes with iron metabolism shows promise as a treatment for P. aeruginosa infections.

Siderophore-based iron acquisition and pathogen control

High-affinity iron acquisition is mediated by siderophore-dependent pathways in the majority of pathogenic and nonpathogenic bacteria and fungi. Considerable progress has been made in characterizing and understanding mechanisms of siderophore synthesis, secretion, iron scavenging, and siderophore-delivered iron uptake and its release. The regulation of siderophore pathways reveals multilayer networks at the transcriptional and posttranscriptional levels. Due to the key role of many siderophores during virulence, coevolution led to sophisticated strategies of siderophore neutralization by mammals and (re)utilization by bacterial pathogens. Surprisingly, hosts also developed essential siderophore-based iron delivery and cell conversion pathways, which are of interest for diagnostic and therapeutic studies. In the last decades, natural and synthetic compounds have gained attention as potential therapeutics for iron-dependent treatment of infections and further diseases. Promising results for pathogen inhibition were obtained with various siderophore-antibiotic conjugates acting as "Trojan horse" toxins and siderophore pathway inhibitors. In this article, general aspects of siderophore-mediated iron acquisition, recent findings regarding iron-related pathogen-host interactions, and current strategies for iron-dependent pathogen control will be reviewed. Further concepts including the inhibition of novel siderophore pathway targets are discussed.

The rare earth, scandium, causes antibiotic overproduction inStreptomycesspp

Despite their importance in the chemical industry, the significance of rare earths in biology has been largely overlooked. Here, it is reported that the rare earth, scandium (Sc), causes antibiotic overproduction by 2-25-fold when added at a low concentration (10-100 microM) to cultures of Streptomyces coelicolor A3(2) (actinorhodin producer), Streptomyces antibioticus (actinomycin producer), and Streptomyces griseus (streptomycin producer). Not just for enhancement of antibiotic production, scandium was also effective in activating the dormant ability to produce actinorhodin in Streptomyces lividans. The effects of scandium were exerted at the level of transcription of pathway-specific positive regulatory genes, as demonstrated by marked up-regulation of actII-ORF4 in S. coelicolor cells exposed to this element. The bacterial alarmone, guanosine 5'-diphosphate 3'-diphosphate, was essential for actinorhodin overproduction provoked by scandium.

Enhancement of copper toxicity by siderophores in Bacillus megaterium

Chelation of copper by siderophores enhanced the toxicity of copper for Bacillus megaterium. Although this antibacterial activity appeared to be rapidly bactericidal, it could be partly reversed by addition of deferrisiderophore , or of ferrisiderophore at high concentration, immediately after exposure of cells to the cupric-siderophore complex.