Singlet Oxygen in Antimicrobial Photodynamic Therapy: Photosensitizer-Dependent Production and Decay in E. coli

Abstract

Several families of photosensitizers are currently being scrutinized for antimicrobial photodynamic therapy applications. Differences in physical and photochemical properties can lead to different localization patterns as well as differences in singlet oxygen production and decay when the photosensitizers are taken up by bacterial cells. We have examined the production and fate of singlet oxygen in Escherichia coli upon photosensitization with three structurally-different cationic photosensitizers, namely New Methylene Blue N (NMB), a member of the phenothiazine family, ACS268, a hydrophobic porphyrin with a single cationic alkyl chain, and zinc(II)-tetramethyltetrapyridinoporphyrazinium salt, a phthalocyanine-like photosensitizer with four positive charges on the macrocycle core. The kinetics of singlet oxygen production and decay indicate different localization for the three photosensitizers, whereby NMB appears to localize in an aqueous-like microenvironment, whereas ACS268 localizes in an oxygen-shielded site, highly reactive towards singlet oxygen. The tetracationic zinc(II) tetrapyridinoporphyrazine is extensively aggregated in the bacteria and fails to produce any detectable singlet oxygen.

Potentiation of photoinactivation of Gram-positive and Gram-negative bacteria mediated by six phenothiazinium dyes by addition of azide ion

“…Nevertheless the majority of authors maintain that a Type II (singlet oxygen) mechanism is the overwhelmingly more important ROS in PDT and especially in antimicrobial PDI 23, 24 . Antimicrobial PDI is an attractive biological area to study the different photochemical mechanisms involved in PDT, as the bacterial cells are particularly sensitive to membrane damage from ROS generated in solution.…”

Reactive oxygen species: Reactions and detection from photosynthetic tissues

“…Measurements of 1270 nm luminescence kinetics from animal cells have, however, yielded longer but extremely variable lifetimes, ranging from 0.4 to 10 µs in cell suspensions or in individual cells (Table 2). Ragas et al [66] estimate that the lifetime of 1 O 2 inside Escherichia coli cells is only 7 ns. The large variation may partially depend on differences between the samples, as the concentrations of substances reacting with 1 O 2 may vary between cell types.…”

A Comprehensive Tutorial onIn VitroCharacterization of New Photosensitizers for Photodynamic Antitumor Therapy and Photodynamic Inactivation of Microorganisms

“…With respect to 1 O 2 detection, the use of the 1270 nm 1 O 2 → 3 O 2 luminescence in both time and spatially resolved experiments has, without doubt, been the most beneficial and informative tool. Several research groups were able to detect singlet oxygen luminescence in lipids and even in living cells/microorganisms after incubation with an exogenous photosensitizer and an optical excitation at different wavelengths [41–44]. …”