Targets of photodyamic inactivation in fungal cells

Photodynamic inactivation (PDI) has been reported to be effective to eradicate a wide variety of pathogens, including antimicrobial-resistant microorganisms. However, there are conflicting reports in the literature about the effect of growth phase on the susceptibility to PDI. The aim of this study was to identify the potential molecular targets of PDI on Candida albicans in exponential growth phase after PDI mediated by methylene blue (50μM) and exposure to a 660nm-LED (P=360mW). For this task, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR) techniques were employed. Pre-irradiation time was set at 10min and exposure time was 15 min delivering a radiant exposure of 162 J/cm2 on a 24-well plate of about 2 cm2. Morphological analysis revealed cell damage after PDI. FT-IR predominantly showed degradation of functional groups related to C-O of deoxyribose; C-C of DNA; C-O stretching vibration of C-OH group of ribose-RNA; P-O stretching modes from the phosphodiester groups of nucleic acids; C=C, C=N, C=O, N=H proteins and amides. Previous studies from our group had demonstrated different targets on the same cells but in stationary growth phase. Therefore, we can conclude that PDI promoted damage to intracellular structures in fungal cells at exponential-phase growth and information on the susceptibility of different growth phases to PDI can be of great importance for the development of treatment strategies that would lead to inactivation of fungal cells in all possible phases of growth in a way that would turn the clinical PDI treatment effective and predictable.