Visible Light Alone Does Not Inactivate Candida albicans

C andida albicans is the most common opportunistic fungal pathogen of humans that is found within the endogenous microflora of healthy individuals. This fungus causes a wide range of human diseases worldwide, ranging from superficial mucosal infections to life-threatening invasive candidiasis in immunocompromised patients. Although new antimycotic drugs are widely available, the widespread use of topical and systemic antifungals has resulted in the development of resistance by C. albicans. Therefore, the search for alternative methods to manage drug-resistant and recurrent candidiasis has become essential.

In the present editorial, I would like to discuss new light-based antifungal treatments such as visible light alone, near infrared (NIR), and IR lasers, metal oxides nanoparticles (NPs), and photodynamic antimicrobial chemotherapy (PAC) treatment. I also introduce a quite new idea to improve the PAC treatment.

During the past two decades, laser irradiation in the visible range, at energy doses in the range of 1–50 J/cm², has been suggested for eliminating various pathogens including C. albicans. ¹ The fungicidal effect was attributed to light-induced reactive oxygen species (ROS) generated by endogenous photosensitizers in the fungi.

Unfortunately, many attempts to repeat these results with visible light only were less successful. ² Light/laser-based devices in the NIR and IR region such as Nd-YAG and CO₂ lasers have also been introduced for treating fungal infections. For example, PinPointe Laser Technology and Noveon use lasers in the NIR (870 and 930 nm) for nail fungi treatment. Most of these laser treatments use high-power light/laser devices that kill the fungi by heat. These laser devices are not recommended since they heat the nail and burn the nail bed.

A new attempt to destroy bacteria is the use of ceramic nanopowders of metal oxides such as ZnO. We examined the antifungal activity of such ZnO NPs with and without visible light illumination against the pathogenic yeast C. albicans and found that they have a marked activity against C. albicans, the antifungal activity being mediated by ROS. ³ Exciting the ZnO NPs by visible light, increased the yeast cell death. ³ Despite the promising results demonstrated, the use of metal oxide NPs in vivo is still not recommended. ⁴

I would like to discuss an alternative approach for killing pathogens, which is the PAC treatment. This technology comprises combination of a photosensitizing (PS) drug and visible light. In this method, an external photosensitizer is introduced into the bacteria or fungi that is then irradiated with visible laser light at the absorption peaks of the photosensitizer. The ROS generated by the illuminated photosensitizer are found to be responsible for killing the pathogens. There are now several PS drugs confirmed by the FDA to be used clinically. This photochemical treatment is being used on various fungi and bacteria, its main disadvantage being the difficulty to introduce the photosensitizer into the tissue and the pathogens. To overcome the limited penetration of the PS, NPs of the PS were prepared. ⁵ An increased penetration depth of NPs within the tissue has been measured using a new noninvasive optical technique. ⁶ For example, the penetration depth into the skin of nano-PS particles was found to be much greater than that measured for the intact PS. ⁶ Moreover, lowering the size of a PS has an additional advantage because when the particle size decreases, its surface area increases, leading to a greater photosensitization activity per given mass compared with larger particles. We can, therefore, conclude that the use of nano-PS combined with visible light could improve dramatically the PAC treatment.