Polarization‑Controlled Photodynamic Activity of Methylene Blue Against Staphylococcus aureus and Escherichia coli

Document Type : Original Article

Authors

1 Department of Biology, CT.C, Islamic Azad University, Tehran, Iran

2 Department of physics, CT.C, Islamic Azad University, Tehran, Iran

Abstract

Photodynamic inactivation (PDI) is a promising antimicrobial strategy that utilizes a non-toxic photosensitizer (PS), molecular oxygen, and light of a specific wavelength to generate cytotoxic reactive oxygen species (ROS). Methylene blue (MB), a phenothiazinium salt with an absorption maximum around 664 nm, is a well-characterized PS that functions via both Type I (radical-forming) and Type II (singlet oxygen-forming) photochemical mechanisms. This study investigates the PDI efficacy of MB-mediated against representative Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria, with a specific focus on the influence of light polarization. MB-mediated PDI was most effective under linearly polarized light in our experimental system compared with circularly polarized or non-polarized light at equivalent fluence rates. The study also confirmed a significant difference in susceptibility between the two bacterial species. S. aureus demonstrated markedly higher susceptibility to MB-mediated PDI than E. coli. This is attributed to the structural disparity in their cell envelopes: the thick, porous peptidoglycan layer of S. aureus facilitates greater uptake and binding of cationic MB. In contrast, the lipopolysaccharide-rich outer membrane of E. coli acts as a formidable permeability barrier, limiting MB access to critical cellular targets. These findings suggest that manipulating light polarization can enhance the bactericidal effects of PDI, paving the way for advances in antimicrobial therapy.

Keywords

Pioneering Advances in Materials
Volume 1, Issue 4
October 2026
Pages 18-27

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