Metal-Oxide Nanoparticles Increase the Bactericidal Effect of Blue Light

B ecause the effectiveness of antibiotic treatment is decreasing as a result of the development of resistant strains, alternative approaches for destroying microorganisms are needed. We would like to draw the attention of the reader to new light-based technologies that might be effective for pathogen inactivation.

Recently, several reports have appeared describing the bactericidal effect of visible light, most of them claiming the blue part (400–500 nm) of the spectrum to be responsible for eliminating various pathogens. For example Feuerstein et al., ^1,2 showed that broadband blue light sources at 400–500 nm exert a phototoxic effect on Porphyromonas gingivalis and Fusobacterium.nucleatum, and Henry et al., ³ demonstrated that argon laser irradiation (488–514 nm) exerts a phototoxic effect on Porphyromonas and Prevotella spp., which are both Gram-negative anaerobic bacteria that produce porphyrins. Propionibacterium acnes were also inactivated by blue light without an exogenous photosensitizer. ^4,5 Investigations using a high-intensity xenon lamp ⁶ demonstrated the sensitivity of Staphylococcus aureus (a nonpigmented bacterium) to visible light, and also identified the bactericidal wavelengths inducing maximum visible-light inactivation. Their results demonstrated that inactivation is optimal using blue light (400–420 nm). Enteric bacterial species and Helicobacter pylori were also found to be sensitive to visible light illumination. ^{7 –9}

The bactericidal effect was attributed to light-induced production of reactive oxygen species (ROS) generated by endogenous photosensitizers in the bacteria. ^2,10,11 Therefore, only bacteria that are rich in endogenous photosensitizers and possessing low antioxidant levels are expected to be sensitive to intense visible light irradiation. In addition, under some conditions, low-intensity visible light might enhance bacterial proliferation, as is the case for mammalian cells. Because different bacteria react differently to light, ¹⁰ the use of visible light for bacterial inactivation might be risky, unless it is used at high fluences. This could also be problematic, as high intensity light illumination is harmful to the surrounding healthy host tissue.

To improve the bactericidal effect of visible light, the introduction of external photosensitizers into the bacteria prior to irradiation was suggested, ^{12 –14} thereby reducing the light fluency required for ROS generation. This photochemical treatment (photodynamic therapy [PDT]) has been tested successfully in vitro. ¹⁵ However, its main disadvantage is the difficulty of specifically introducing photosensitizers into bacteria. ¹⁵

We therefore suggest a novel approach for enhancing the light-mediated killing of pathogens, by introducing metal oxide nanoparticles (np) as photosensitizers. In recent years, nanomaterials have gained special attention because of their higher chemical activity, as compared with that of the same compounds in their bulk form. ^{16 –18} Of particular interest are nano–titanium dioxide (TiO₂) and zinc oxide (ZnO), which readily penetrate into the bacterial cell. Water suspensions of ZnO and (TiO₂) np were found to produce stable oxy radicals. ^{19 –21} Moreover, a remarkable enhancement of ROS was found following illumination of ZnO and TiO₂ nanoparticles with blue light. In the case of ZnO, ²² blue light increased the amount of OH by a factor of 4. ²⁰ It therefore appears that metal oxide np that readily penetrate bacteria, combined with visible light irradiation, might serve as a valuable approach for elimination of bacteria. Indeed, a recent study demonstrated that incubation of S. aureus or Staphylococcus epidermitis with ZnO or TiO₂ np results in a reduction by 80–90% of bacterial viability following low-fluence 415 nm light irradiation, at which light alone had almost no effect. ²²

Metal oxides used on a micro scale have a rich history, with applications in food, materials and biological studies; but, as asbestos toxicity has taught us, the shape, size, and morphology of a compound can also play a significant role in its biotoxicity. For example, TiO₂ has been previously classified as biologically inert, both in animals and in humans; ^23,24 therefore, it was considered very safe. However, there have been several recent studies claiming that metal oxide nanoparticles that easily penetrate the skin can cause adverse effects on organ, tissue, cellular, and subcellular levels because of their unusual physicochemical properties, for example,,small size, high surface area to volume ratio, electronic properties, surface structure reactivity, and aggregation behavior. ^25,26

To avoid the possible toxic effects of metal oxide np on healthy tissues, it is possible to coat the np on the surface of various substrates, including ceramics and polymers. ^{19,27 –29} Recently, ZnO np were synthesized and deposited on the surface of cotton fabrics using ultrasound irradiation. Optimization of the process resulted in a homogeneous distribution of ZnO nanocrystals on the fabric surface. The antibacterial activities of the ZnO-fabric composite were tested against Escherichia coli (Gram negative) and S. aureus (Gram positive) cultures. A significant bactericidal effect was demonstrated. ³⁰ Dynamic light scattering and TEM studies did not reveal the presence of any nanoparticles in the leaching solution. This suggests that the sonochemically deposited ZnO np are strongly attached to the textile substrate ³⁰ and would be unlikely to penetrate the human body.

It therefore appears that visible light, combined with metal oxide np coated on textiles, could serve as a valuable and novel method for disinfecting wounds. For example, we are now trying to treat infected wounds with transparent bandages coated with ZnO applied prior to irradiation. In this technology, the coated metal oxide np serve as photosensitizers, therefore enabling the use of low fluences of light that are nontoxic to the healthy tissues.

We expect this new technology based on bandages coated with metal oxide np and visible light to be effective for wound sterilization.