Antimicrobial Photodynamic Therapy Reveals Differential Species Susceptibility and Ecological Shifts in
                    <i>Candida albicans–Pseudomonas aeruginosa</i>
                    Mixed Model

doi:10.1177/25785478261481597

DOI: 10.1177/25785478261481597 ISSN: 2578-5478

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

Jiahe Li, Liang Zhou

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/cm ² ) 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 log ₁₀ (≈99%) at 60 J/cm ² . 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.