Photoinactivation of fluconazole-resistant Candida albicans and Candida glabrata

Objectives: This study evaluated whether photodynamic therapy (PDT) would have similar efficacy in killing a standard reference (ATCC) and three fluconazole-resistant (R) strains of Candida albicans (2327ATCC, 10R, 15R and 63R) and Candida glabrata (2001ATCC, 50R, 63R and 87R). Methods: The photosensitizer (PS) used in this study was Photogem^® (hematoporphyrin derivative). A light emitting diode (LED/~627nm) device was used to excite the PS. Suspensions of each Candida strain (10⁶ viable cells/milliliter) were treated with three PS concentrations (50, 100 and 150mg/l) and exposed to 50, 75 and 100J/cm² LED light fluences (n=3 each). Control suspensions were treated only with PS concentrations, only exposed to the LED light fluences or not exposed to neither LED light nor PS. From each sample, serial dilutions were obtained and aliquots of 25µl were plated in triplicate on Sabouraud Dextrose Agar. After plates incubation (37°C for 48h), colonies were counted (CFU/ml) and the data were statistically analyzed with ANOVA and the Tukey's test (p<0.05). Results: Complete killing of all C. albicans and C. glabrata strains was observed under some experimental conditions. Compared to ATCC, R strains required higher PS concentrations for its complete inactivation. A similar susceptibility to PDT between ATCC and R strains was only observed for C. albicans, after photosensitization with 50mg/L and subsequent illumination at 100J/cm². It was verified a significant variation among strains of the same species in how they responded to PDT, despite the fluconazole-resistance. Control conditions showed no significant changes in cell numbers throughout the course of the experiments. Conclusions: The photoactivation of Photogem^® by LED light proved to be effective for the inactivation of R and ATCC strains of C. albicans and C. glabrata. However, C. albicans R and C. glabrata R showed less susceptibility to PDT, in comparison to standard strains. Grant: FAPESP 2005/03226-3.