Photodynamic inactivation of Candida albicans and Candida tropicalis with aluminum phthalocyanine chloride nanoemulsion

Abstract Candida albicans and Candida tropicalis are commensal yeasts able to cause human infections. The increased incidence of Candida spp. infections and fungal resistance to conventional antifungal drugs are recurring problems. Antimicrobial Photodynamic Treatment (APDT) is a promising alternative that uses a photosensitizer (PS), which when activated by light in the presence of oxygen can lead to microbial inactivation. The in vitro susceptibilities of C. albicans and C. tropicalis to APDT with aluminum phthalocyanine chloride in nanoemulsion (ClAlPc/NE) in combination with red light were investigated. PS concentration- and fluence-dependent cell survival after APDT were compared before and after unbound extracellular PS had been washed out. The PS uptake and its subcellular localization were also determined by confocal fluorescence microscopy employing DAPI and Calcofluor staining. Exposure to light in the absence of the PS and treatment with the PS in the absence of light did not kill the fungi. Cells were killed in a fluence-dependent manner. APDT with ClAlPc/NE (0.045 μM and 100 J cm-2) resulted in a reduction of five orders of magnitude in viability for C. albicans and between four and five orders of magnitude for C. tropicalis. Washing the cells to remove unbound PS before light exposure did not impair fungal inactivation, suggesting that cell photosensitization was mainly carried out by cell bound ClAlPc. The degree of ClAlPc uptake was dependent on its concentration. Internalization of ClAlPc by C. albicans and C. tropicalis was confirmed by confocal fluorescence microscopy that showed the PS does not penetrate the nucleus and instead accumulates in specific regions of the cytoplasm. Our results show that incorporating the water-insoluble ClAlPc into a nanoemulsion leads to an efficient formulation capable of photoinactivating both Candida species.