Xylan-Porphyrin Hydrogels as Light-Triggered Gram-Positive Antibacterial Agents

In Situ Single-Cell Bacterial Imaging Provides Mechanistic Insight into the Photodynamic Action of Photosensitizer-Loaded Hydrogels

Hydrogels, three-dimensional hydrophilic polymeric networks with high water retaining capacity, have gained prominence in wound management and drug delivery due to their tunability, softness, permeability, and biocompatibility. Electron-beam polymerized poly(ethylene glycol) diacrylate (PEGDA) hydrogels are particularly useful for phototherapies such as antimicrobial photodynamic therapy (aPDT) due to their excellent optical properties. This work takes advantage of the transparency of PEGDA hydrogels to investigate bacterial responses to aPDT at the single-cell level, in real-time and in situ. The photosensitizer methylene blue (MB) was loaded in PEGDA hydrogels by using two methods: reversible loading and irreversible immobilization within the 3D polymer network. MB release kinetics and singlet oxygen generation were studied, revealing the distinct behaviors of both hydrogels. Real-time imaging of Escherichia coli was conducted during aPDT in both hydrogel types, using the Min protein system to report changes in bacterial physiology. Min oscillation patterns provided mechanistic insights into bacterial photoinactivation, revealing a dependence on the hydrogel preparation method. This difference was attributed to the mobility of MB within the hydrogel, affecting its direct interaction with bacterial membranes. These findings shed light on the complex interplay between hydrogel properties and the bacterial response during aPDT, offering valuable insights for the development of antibacterial wound dressing materials. The study demonstrates the capability of real-time, single-cell fluorescence microscopy to unravel dynamic bacterial behaviors in the intricate environment of hydrogel surfaces during aPDT.

Visible light-activated photosensitizer inhibits the plasmid-mediated horizontal gene transfer of antibiotic resistance genes

Inhibition of plasmid transfer, including transformation and conjugation, is essential to prevent the spread of plasmid-encoded antimicrobial resistance. Photosensitizers have been successfully used in the treatment of serious infectious diseases, however, the effects of photosensitizers on the plasmid transfer are still elusive. In this study, we determined the transformation and conjugation efficiency of plasmid pUC19 and pRP4, respectively, when exposed to a photosensitizer (Visible Light-activated Rose Bengal, VLRB). The results showed that the activation of VLRB resulted in up to a 580-fold decrease in the transformation frequency of pUC19 and a 10-fold decrease in the conjugation frequency of pRP4 compared with the non-VLRB control. The inhibition of pUC19 transformation by VLRB exhibited a dose-dependent manner and was attributed to the changes in the plasmid conformation. The inhibition of pRP4 conjugation was associated with the generation of extracellular free radicals, induced oxidative stress, suppression of the mating pair formation gene (trbBp) and DNA transfer and replication gene (trfAp), and enhanced expression of the global regulatory genes (korA, korB, and trbA). These findings highlight the potential of visible light-activated photosensitizer for mitigating the dissemination of plasmid-encoded antibiotic resistance genes.