Potentiation of curcumin-mediated antimicrobial photodynamic therapy by sub-inhibitory concentrations of essential oils

Abstract Antimicrobial photodynamic therapy (aPDT) is an emerging alternative to conventional antimicrobials; however, its efficacy against Gram-negative bacteria with complex membrane structures remains limited. In this study, we investigated the ability of essential oils to potentiate curcumin-mediated aPDT while avoiding confounding effects from intrinsic oil toxicity. Thyme ( Thymus vulgaris ) and pine ( Pinus sylvestris ) essential oils were chemically characterized by GC–MS and applied at strictly sub-inhibitory concentrations to isolate true photodynamic contributions. Singlet oxygen ( 1 O 2 ) generation by curcumin under blue-light irradiation was confirmed using a DPBF-based assay, establishing its photodynamic competence. Antimicrobial photodynamic inactivation experiments were performeda against methicillin-resistant Staphylococcus aureus (MRSA, ATCC 43300) and Escherichia coli ( E. coli , ATCC 8739). Curcumin-mediated PDT alone produced limited bacterial killing, whereas the addition of essential oils resulted in pronounced, fluence-dependent multi-log reductions in viable counts. Notably, essential oil–assisted PDT achieved up to a 4.02 log 10 reduction, exceeding standard disinfection thresholds, even in Gram-negative E. coli , which is typically resistant to hydrophobic photosensitizers. The enhanced photodynamic efficacy is attributed to essential oil–induced modulation of bacterial membrane permeability, facilitating photosensitizer access and increasing susceptibility to reactive oxygen species. By operating at sub-MIC oil concentrations, this approach minimizes dark toxicity and clearly distinguishes photodynamic potentiation from additive antimicrobial effects. These findings highlight the potential of essential oils as non-antibiotic adjuvants for improving the efficacy and spectrum of antimicrobial photodynamic therapy.