Plant-derived efflux pump inhibitors potentiate Chlorin e6-based antimicrobial photodynamic therapy against Bacillus cereus and Escherichia coli

ANTIMICROBIAL RESISTANCE: especially via efflux pump-mediated multidrug resistance is a major factor limiting antibacterial therapy efficacy. Antimicrobial photodynamic therapy (aPDT) is a non-antibiotic strategy that overcomes resistance through generation of reactive oxygen species. Plant-derived compounds exhibit efflux pump inhibitory activity and potential photosensitizing capabilities. This study evaluated the use of plant-derived efflux pump inhibitors (EPIs): berberine, capsaicin, coumarin 6, curcumin, palmatine, and piperine in combination with Chlorin e6 and aPDT against Bacillus cereus (Gram-positive) and Escherichia coli (Gram-negative). The aim was to enhance Chlorin e6 accumulation and reduce bacterial dry mass density. Both qualitative and quantitative methods were used in the study. Minimum inhibitory concentrations (MICs) of EPIs were determined to guide treatment concentrations. A modified resazurin assay assessed bacterial viability after 24-h incubation with EPIs. Spectroscopic properties of all compounds were recorded using UV-VIS spectroscopy. Digital holotomography (DHT) was applied to monitor changes in refractive index (RI) of bacterial cells induced by incubation and treatment, and results were validated using confocal fluorescence microscopy. Changes in RI values indicated structural and compositional changes in the bacteria. Naturally derived EPIs effectively potentiated aPDT by promoting photosensitizer uptake and induced a measurable reduction in dry mass density. Treatment efficacy using combination therapy for B.cereus and E.coli improved 273.90 % and 227.72 %, respectively. These findings suggest naturally derived EPIs effectively potentiate Chlorin e6-mediated aPDT and improve bactericidal capacity against bacteria. The study offers mechanistic insight into photodynamic enhancement via plant-based inhibitors and supports potential applications in clinical antimicrobial therapy, food safety, and industrial decontamination.