Photodynamic damage predominates on different targets depending on cell growth phase of Candida albicans.

Photodynamic inactivation (PDI) has been reported to be effective to eradicate a wide variety of pathogens, including antimicrobial-resistant microorganisms. The aim of this study was to identify the potential molecular targets of PDI depending on growth phase of Candida albicans. Fungal cells in lag (6h) and stationary (48h) phases were submitted to PDI mediated by methylene blue (MB) combined with a (662±21) nm-LED, at 360mW of optical power. Pre-irradiation time was 10min and exposure times were 12min, 15min and 18min delivering radiant exposures of 129.6J/cm2, 162J/cm2 and 194.4J/cm2, respectively, on a 24-well plate of about 2cm2 at an irradiance of 180mW/cm2. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force spectroscopy (AFS) and Fourier transform infrared spectroscopy (FT-IR) were employed to evaluate the photodynamic effect in young and old fungal cells following 15min of irradiation. Morphological analysis revealed wrinkled and shrunk fungal cell membrane for both growth phases while extracellular polymeric substance (EPS) removal was only observed for old fungal cells. Damaged intracellular structures were more pronounced in young fungal cells. The surface nanostiffness of young fungal cells decreased after PDI but increased for old fungal cells. Cellular adhesion force was reduced for both growth phases. Fungal cells in lag phase predominantly showed degradation of nucleic acids and proteins, while fungal cells in stationary phase showed more pronounced degradation of polysaccharides and lipids. Taken together, our results indicate different molecular targets for fungal cells in lag and stationary growth phase following PDI.