Antimicrobial photodynamic therapy reprograms resistance by inducing sensitization in methicillin-resistant Staphylococcus aureus.

Combination therapies are a crucial strategy to decelerate the progression of antimicrobial resistance in microorganisms. In this context, antimicrobial photodynamic therapy (aPDT) emerges as a valuable adjuvant, not only for infection eradication but also for its potential to modulate the pharmacodynamics of antimicrobials in resistant bacteria. To evaluate whether aPDT can induce a sustained increase in the susceptibility of methicillin-resistant Staphylococcus aureus (MRSA) to antibiotics, characterize a possible phenotypic memory effect, and assess whether this altered phenotype influences intracellular survival in macrophages. MRSA strains were treated with curcumin (10 µM) and blue light (450 nm, 5-20 J/cm²) in sequential cycles with 8-hour intervals. The susceptibility to azithromycin (AZY), sulfamethoxazole-trimethoprim (SXT), and oxacillin (OXA) were evaluated through MIC assays. The persistence of sensitization was monitored for 96 h in the absence of selective pressure. Reapplication of aPDT cycles was performed after bacterial regrowth. In parallel, intracellular infectivity was assessed using RAW 264.7 macrophages infected with treated or untreated MRSA strains. aPDT increased MRSA susceptibility to antibiotics by up to 37-fold. This sensitization persisted for up to 96 h after treatment, even in the absence of antibiotic pressure, suggesting a phenotypic memory. Repeated cycle of aPDT after regrowth reestablished susceptibility. Treated MRSA strains also exhibited reduced intracellular survival in macrophages, indicating impaired virulence. aPDT not only enhances antibiotic susceptibility in resistant S. aureus but also promotes a reversible, phenotypic memory and reduces intracellular infectivity. These findings position aPDT as a promising strategy for controlling resistant infections by reprogramming bacterial behavior beyond immediate inactivation.