Rational Control of Combined Photothermal and Photodynamic Therapy for Effective Eradication of Biofilms

Abstract

New therapies are essential for eliminating antibiotic-resistant bacteria and their biofilms, which is a major global health threat causing millions of deaths annually. Here, we demonstrate a combination of photodynamic therapy (PDT) and photothermal therapy (PTT) for the inhibition of biofilms of Pseudomonas aeruginosa and Staphylococcus epidermidis using Aminolaevulinic acid (ALA)-loaded Polyacrylic acid coated superparamagnetic iron oxide nanoparticles (PAA-SPIONs) at 200, 600 and 1000 µg/mL Fe concentrations under 640 nm (0.75 W/cm2), 808 (2.6 W/cm2) and 640+808 nm irradiations (0.75 + 2.6 W/cm2, 20 min). PTT experiments indicate ALA/PAA-SPION concentration-dependent heating up to 10.2°C for PAA-SPIONs and 9.3°C for ALA/PAA-SPIONs under combined 640+808 nm laser excitations. Bacterial growth inhibition of ALA/PAA-SPIONs was investigated with and without laser irradiations for 10 min using 150 and 600 µg Fe/mL or 0.5 mM and 2 mM ALA on both bacterial types. These experiments indicate 3 to 6-log reduction of P. aeruginosa compared to control samples (without nanoparticle or laser) for increasing Fe and ALA concentrations. Growth was completely inhibited by ALA/PAA-SPIONs at 640+808 nm irradiation. ALA/PAA-SPIONs provided growth inhibition of S. epidermidis between 2-log to 4-log for increasing wavelengths, Fe and ALA doses. PAA-SPIONs and laser together inhibited biofilms of P. aeruginosa with 3 to 11-log reductions at increasing laser wavelengths. The reduction of the biofilm with ALA/PAA-SPIONs and laser reaches 8-log for 640 nm and 13-log for 808 nm excitations. We accurately model the wavelength, time, and nanoparticle concentration dependence of PTT for the first time. These results pave the way for effective PDT/PTT elimination of biofilms of P. aeruginosa and S. epidermidis.