Anat Lipovsky

Understanding the Antibacterial Mechanism of CuO Nanoparticles: Revealing the Route of Induced Oxidative Stress

To date, there is still a lack of definite knowledge regarding the interaction of CuO nanoparticles with bacteria and the possible permeation of the nanoparticles into bacterial cells. This study was aimed at shedding light on the size-dependent (from the microscale down to the small nanoscale) antibacterial activity of CuO. The potent antibacterial activity of CuO nanoparticles was found to be due to ROS-generation by the nanoparticles attached to the bacterial cells, which in turn provoked an enhancement of the intracellular oxidative stress. This paradigm was confirmed by several assays such as lipid peroxidation and reporter strains of oxidative stress. Furthermore, electron microscopy indicated that the small nanoparticles of CuO penetrated the cells. Collectively, the results reported herein may reconcile conflicting concepts in the literature concerning the antibacterial mechanism of CuO nanoparticles, as well as highlight the potential for developing sustainable CuO nanoparticles-based devices for inhibiting bacterial infections.

Antifungal activity of ZnO nanoparticles?the role of ROS mediated cell injury

Metal oxide nanoparticles have marked antibacterial activity. The toxic effect of these nanoparticles, such as those comprised of ZnO, has been found to occur due to an interaction of the nanoparticle surface with water, and to increase with a decrease in particle size. In the present study, we tested the ability of ZnO nanoparticles to affect the viability of the pathogenic yeast, Candida albicans (C. albicans). A concentration-dependent effect of ZnO on the viability of C. albicans was observed. The minimal fungicidal concentration of ZnO was found to be 0.1 mg ml(-1) ZnO; this concentration caused an inhibition of over 95% in the growth of C. albicans. ZnO nanoparticles also inhibited the growth of C. albicans when it was added at the logarithmic phase of growth. Addition of histidine (a quencher of hydroxyl radicals and singlet oxygen) caused reduction in the effect of ZnO on C. albicans depending on its concentration. An almost complete elimination of the antimycotic effect was achieved following addition of 5 mM of histidine. Exciting the ZnO by visible light increased the yeast cell death. The effects of histidine suggest the involvement of reactive oxygen species, including hydroxyl radicals and singlet oxygen, in cell death. In light of the above results it appears that metal oxide nanoparticles may provide a novel family of fungicidal compounds.

Eradication of Multi‐Drug Resistant Bacteria by a Novel Zn‐doped CuO Nanocomposite

Zinc-doped copper oxide nanoparticles are synthesized and simultaneously deposited on cotton fabric using ultrasound irradiation. The optimization of the processing conditions, the specific reagent ratio, and the precursor concentration results in the formation of uniform nanoparticles with an average size of ≈30 nm. The antibacterial activity of the Zn-doped CuO Cu₀.₈₈Zn₀.₁₂O in a colloidal suspension or deposited on the fabric is tested against Escherichia coli (Gram negative) and Staphylococcus aureus (Gram positive) bacteria. A substantial enhancement of 10,000 times in the antimicrobial activity of the Zn-CuO nanocomposite compared to the pure CuO and ZnO nanoparticles (NPs) is observed after 10 min exposure to the bacteria. Similar activities are observed against multidrug-resistant bacteria (MDR), (i.e., Methicillin-resistant S. aureus and MDR E. coli) further emphasizing the efficacy of this composite. Finally, the mechanism for this enhanced antibacterial activity is presented.

Visible light‐induced killing of bacteria as a function of wavelength: Implication for wound healing

Visible light (400–800 nm) at high intensity was previously found to kill bacteria that are frequently found in infected wounds, while low‐power white light enhances bacterial proliferation. The phototoxic effect was found to involve induction of reactive oxygen species (ROS) production by the bacteria. The aim of the present study was to identify the most effective wavelengths in the visible range for inducing a bactericidal effect.

Sensitivity of Staphylococcus aureus Strains to Broadband Visible Light

The phototoxic effect of illumination with broadband visible light on the viability of two Staphylococcus aureus strains was examined in the present study. A difference in the light sensitivity of the two strains was found. Illumination of the tested strains with a fluence rate of 180 J cm−2 caused a reduction of up to 99.8% in the colony count of one of the strains (the “sensitive” strain). Illumination of the other strain (the “resistant” strain) resulted in a 55.5% reduction in viability. Proliferation of both strains was observed at low fluence rates of light. The phototoxic effect was found to be dependent on oxy radical production. The light‐sensitive strain produced higher amounts of hydroxyl and superoxide radicals than the “resistant” strain. Adaptation to oxidative stress was exhibited only by the “resistant” strain. The “sensitive” strain produced ten times more endogenous porphyrins and secreted almost nine times more porphyrins than the resistant strain. Furthermore, the “resistant” strain produced twice as many carotenoids that protect the strain from illumination than the “sensitive” strain. These results indicate that high intensities of visible light cause bacterial photoeradication, a reaction which may assist wound healing by killing the infecting bacteria. On the other hand, low intensities of white light were found to enhance bacterial proliferation and thus prolong wound infection.

A possible mechanism for visible light‐induced wound healing

Chronic wounds resistant to conventional therapy have been treated successfully with low energy lasers and light emitting diodes (LEDs) in the visible and near IR region. It has been proposed that production of low level reactive oxygen species (ROS) following illumination is the first step of photobiomodulation. It was also shown that white light (400–800 nm) has similar stimulatory effects as lasers and LEDs. ROS at higher levels are toxic to cells and bacteria.

The Different Behavior of Rutile and Anatase Nanoparticles in Forming Oxy Radicals Upon Illumination with Visible Light: An EPR Study

Photoexcited TiO2 has been found to generate reactive oxygen species, yet the precise mechanism and chemical nature of the generated oxy species especially regarding the different crystal phases remain to be elucidated. Visible light‐induced reactions of a suspension of titanium dioxide (TiO2) in water were investigated using electron paramagnetic resonance (EPR) coupled with the spin‐trapping technique. Increased levels of both hydroxyl (˙OH) and superoxide anion (˙O2−) radicals were detected in TiO2 rutile and anatase nanoparticles (50 nm). The intensity of signals assigned to the ˙OH and ˙O2− radicals was larger for the anatase phase than that originating from rutile. Moreover, illumination with visible (nonUV) light enhanced ˙O2− formation in the rutile phase. Singlet oxygen was not detected in water suspension of TiO2 neither in rutile nor in anatase nanoparticles, but irradiation of the rutile phase with visible light revealed a signal, which could be attributed to singlet oxygen formation. The blue part of visible spectrum (400–500 nm) was found to be responsible for the light‐induced ROS in TiO2 nanoparticles. The characterization of the mechanism of visible light‐induced oxy radicals formation by TiO2 nanoparticles could contribute to its use as a sterilization agent.

Biocidal properties of TiO2 powder modified with Ag nanoparticles

Silver nanoparticles (NPs) were synthesized sonochemically by the reduction of silver ions with ethylene glycol and simultaneously deposited on different forms of TiO2 powders (commercial Degussa P-25, synthetic anatase and mesoporous titania). The Ag-TiO2 nanocomposites were characterized by X-ray electron diffraction (XRD), transmission electron microscopy (TEM), energy-dispersed X-ray analysis (EDX), UV absorption spectroscopy (UV), Z-potential measurements and electron paramagnetic resonance (EPR). The results demonstrated homogeneous distribution of silver nanoparticles ∼3 nm in size, strongly attached to the surface of titania. The antimicrobial properties of Ag-TiO2 were tested against a number of Gram-positive and Gram-negative bacteria. A high bactericidal effect was found in the absence of UV light. The reduction in bacterial viability was between 3 and 4.2 logs. Based on the experimental data it was concluded that enhanced antimicrobial activity of the Ag-TiO2 originated from both the oxidative stress generated by silver nanoparticles and the presence of silver ions on the surface of the silver-titania composite.

The influence of the crystalline nature of nano-metal oxides on their antibacterial and toxicity properties

The antibacterial properties of nano-metal oxides (ZnO, CuO) are based on the formation of reactive oxygen species (ROS). This work reveals that the antibacterial properties of these nano-metal oxides are strongly dependent on their crystalline structure. The antibacterial activity of the nanooxides was tested against four types of bacteria that commonly cause nosocomial infections. The sonochemical method was applied not only for synthesis of nanooxides but also to their coating on textiles. The antibacterial properties of textiles coated with commercial and sonochemically prepared nano-metal oxides were evaluated and compared. The toxicity was evaluated on human lung cells and amphibian embryos, as representative models for inhalation and aquatic toxicology. The sonochemically prepared metal nanooxides are better antimicrobials than commercially available metal oxides with the same particle size range. It was found that the crystallites which have more defects and less organized structure are more toxic. The formation of ROS was studied by electron spin resonance (ESR) measurements for both the sonochemically prepared and commercial samples of ZnO/CuO nanoparticles. A significant increase in the production of radical species was found in the more defective, sonochemically prepared samples, as compared to the commercial ones. Since modulation of the nanoparticle defects influenced their toxicity, the possibility of engineering safer nano-antibacterials is indicated.

Enhanced inactivation of bacteria by metal-oxide nanoparticles combined with visible light irradiation

In recent years nano‐metaloxides which easily penetrate into the cells with special interest due to their higher chemical reactivity as compared to that of similar materials in the bulk form. Of particular interest are nano‐TiO2 and ZnO, which have been widely used for their bactericidal and anticancerous properties.