Antimicrobial Photodynamic Therapy Reveals Differential Species Susceptibility and Ecological Shifts in Candida albicans-Pseudomonas aeruginosa Mixed Model.

BACKGROUND: Candida albicans and Pseudomonas aeruginosa are frequently co-isolated from refractory endodontic infections. Antimicrobial photodynamic therapy (aPDT) is a promising adjunctive disinfection strategy; however, its efficacy against polymicrobial communities containing structurally distinct organisms remains unclear. OBJECTIVE: To evaluate methylene blue-mediated aPDT at three energy densities (15, 30, and 60 J/cm2) against a dual-species coculture model and to investigate the ecological consequences of differential species susceptibility. METHODS: Dual-species biofilms were established on silicone substrates. Mature cocultures were treated with 100 µM methylene blue and irradiated with 630 nm light-emitting diode (LED) light. Morphological changes were assessed by scanning electron microscopy (SEM); bacterial and fungal viability were quantified by colony forming unit assays on species-selective media. RESULTS: aPDT produced dose-dependent structural damage to P. aeruginosa. SEM showed approximately 90% morphological reduction, and viable counts fell by approximately 1.95 log10 (≈99%) at 60 J/cm2. C. albicans remained morphologically intact across all energy densities. Selective, dose-dependent bacterial inactivation, confirmed by viable counts, was accompanied by secondary fungal overgrowth. C. albicans proliferation rates reached 460-690% within 18 h. CONCLUSIONS: Differential species susceptibility to methylene blue-mediated aPDT disrupts microbial competitive equilibrium in polymicrobial communities. Optimized aPDT protocols for polymicrobial endodontic infections should incorporate antifungal components to prevent secondary fungal proliferation.