Dušan Milivojević

A comparative antimicrobial and toxicological study of gold(III) and silver(I) complexes with aromatic nitrogen-containing heterocycles: synergistic activity and improved selectivity index of Au(III)/Ag(I) complexes mixture

Five aromatic nitrogen-containing heterocycles, pyridazine (pydz, 1), pyrimidine (pm, 2), pyrazine (pz, 3), quinoxaline (qx, 4) and phenazine (phz, 5) have been used for the synthesis of gold(III) and silver(I) complexes. In contrast to the mononuclear Au1–5 complexes all having square-planar geometry, the corresponding Ag1–5 complexes have been found to be polynuclear and of different geometries. Complexes Au1–5 and Ag1–5, along with K[AuCl4], AgNO3 and N-heterocyclic ligands used for their synthesis, were evaluated by in vitro antimicrobial studies against a panel of microbial strains that lead to many skin and soft tissue, respiratory, wound and nosocomial infections. All tested complexes exhibited excellent to good antibacterial activity with minimal inhibitory (MIC) values in the range of 2.5 to 100 μg mL−1 against the investigated strains. The complexes were particularly efficient against pathogenic Pseudomonas aeruginosa (MIC = 2.5–30 μg mL−1) and had a marked ability to disrupt clinically relevant biofilms of strains with high inherent resistance to antibiotics. Moreover, the Au1–4 and Ag1–5 complexes exhibited pronounced ability to competitively intercalate double stranded genomic DNA of P. aeruginosa, which was demonstrated by gel electrophoresis techniques and supported by molecular docking into the DNA major groove. Antiproliferative effect on the normal human lung fibroblast cell line MRC5 has also been evaluated in order to determine therapeutic potential of Au1–5 and Ag1–5 complexes. Since the investigated gold(III) complexes showed much lower negative effects on the viability of the MRC5 cell line than their silver(I) analogues and slightly lower antimicrobial activity against the investigated strains, the combination approach to improve their pharmacological profiles was applied. Synergistic antimicrobial effect and the selectivity index of 10 were achieved for the selected gold(III)/silver(I) complexes mixtures, as well as higher P. aeruginosa PAO1 biofilm disruption activity, and improved toxicity profile towards zebrafish embryos, in comparison to the single complexes. To the best of our knowledge, this is the first report on synergistic activity of gold(III)/silver(I) complexes mixtures and it could have an impact on development of new combination therapy methods for the treatment of multi-resistant bacterial infections.

Magnetic and structural studies of CoFe 2 O 4 nanoparticles suspended in an organic liquid

We present a study of magnetic and structural properties of CoFe2O4 nanoparticles suspended in an organic liquid. Transmission electron microscopy shows that the nanoparticles have a narrow size distribution of average particle size 5.9 ± 1.0 nm. X-ray diffraction shows that the particles are of cubic spinel crystal structure. Dynamic light scattering measurements reveal the existence of an organic shell around the CoFe2O4 nanoparticles with an average hydrodynamic diameter of 14.4 nm. Coercive magnetic field at T = 5K is found to be 11.8 kOe. Disappearance of the coercive field and remanent magnetization at about 170K suggests that the CoFe2O4 nanoparticles are superparamagnetic at higher temperatures which is confirmed by the room temperature Mossbauer spectrum analysis. Saturation magnetization of the nanoparticles of 80.8 emu/g(CoFe2O4) at 5K reaches the value detected in the bulk material and remains very high also at room temperature. The cobalt ferrite nanoparticle system synthesized in this work exhibits magnetic properties which are very suitable for various biomedical applications.

Anti-biofilm Properties of Bacterial Di-Rhamnolipids and Their Semi-Synthetic Amide Derivatives

A new strain, namely Lysinibacillus sp. BV152.1 was isolated from the rhizosphere of ground ivy (Glechoma hederacea L.) producing metabolites with potent ability to inhibit biofilm formation of an important human pathogens Pseudomonas aeruginosa PAO1, Staphylococcus aureus, and Serratia marcescens. Structural characterization revealed di-rhamnolipids mixture containing rhamnose (Rha)-Rha-C10-C10, Rha-Rha-C8-C10, and Rha-Rha-C10-C12 in the ratio 7:2:1 as the active principle. Purified di-rhamnolipids, as well as commercially available di-rhamnolipids (Rha-Rha-C10-C10, 93%) were used as the substrate for the chemical derivatization for the first time, yielding three semi-synthetic amide derivatives, benzyl-, piperidine-, and morpholine. A comparative study of the anti-biofilm, antibacterial and cytotoxic properties revealed that di-Rha from Lysinibacillus sp. BV152.1 were more potent in biofilm inhibition, both cell adhesion and biofilm maturation, than commercial di-rhamnolipids inhibiting 50% of P. aeruginosa PAO1 biofilm formation at 50 μg mL-1 and 75 μg mL-1, respectively. None of the di-rhamnolipids exhibited antimicrobial properties at concentrations of up to 500 μg mL-1. Amide derivatization improved inhibition of biofilm formation and dispersion activities of di-rhamnolipids from both sources, with morpholine derivative being the most active causing more than 80% biofilm inhibition at concentrations 100 μg mL-1. Semi-synthetic amide derivatives showed increased antibacterial activity against S. aureus, and also showed higher cytotoxicity. Therefore, described di-rhamnolipids are potent anti-biofilm agents and the described approach can be seen as viable approach in reaching new rhamnolipid based derivatives with tailored biological properties.

Biofilm-forming ability and infection potential of Pseudomonas aeruginosa strains isolated from animals and humans

Abstract Pseudomonas aeruginosa has been amongst the top 10 ‘superbugs’ worldwide and is causing infections with poor outcomes in both humans and animals. From 202 P. aeruginosa isolates (n = 121 animal and n = 81 human), 40 were selected on the basis of biofilm‐forming ability and were comparatively characterized in terms of virulence determinants to the type strain P. aeruginosa PAO1. Biofilm formation, pyocyanin and hemolysin production, and bacterial motility patterns were compared with the ability to kill human cell line A549 in vitro. On average, there was no significant difference between levels of animal and human cytotoxicity, while human isolates produced higher amounts of pyocyanin, hemolysins and showed increased swimming ability. Non‐parametric statistical analysis identified the highest positive correlation between hemolysis and the swarming ability. For the first time an ensemble machine learning approach used on the in vitro virulence data determined the highest relative predictive importance of the submerged biofilm formation for the cytotoxicity, as an indicator of the infection ability. The findings from the in vitro study were validated in vivo using zebrafish (Danio rerio) embryos. This study highlighted no major differences between P. aeruginosa species isolated from animal and human infections and the importance of pyocyanin production in cytotoxicity and infection ability.

Structural and Magnetic Properties of the Zn-Mn-O System

Zn-Mn-O semiconductor crystallites with nominal manganese concentration x = 0.01, 0.02, 0.04 and 0.10 were synthesized by a solid state reaction route using oxalate precursors. Thermal treatment procedure was carried out in air at different temperatures in the range 400 - 900°C. The samples were investigated by X-ray diffraction, magnetization measurements and electron paramagnetic resonance. X-ray analysis reveals that dominant crystal phase in the Zn-Mn-O system corresponds to the wurtzite structure of ZnO. Room temperature ferromagnetism is observed in the Zn-Mn-O samples with lower manganese concentration, x ≤ 0.04, thermally treated at low temperature (500°C). Saturation magnetization in the sample with x = 0.01 is found to be 0.05 μB/Mn. The ferromagnetic phase seems to be developed by Zn diffusion into Mn-oxide grains.