Photosensitizer-specific bacterial stress responses in Escherichia coli reveal distinct targets in photoinduced inactivation.

The ongoing antibiotic crisis calls for alternative antimicrobial strategies. Antimicrobial photodynamic therapy (aPDT) offers a sustainable option, employing a light-activated photosensitizer (PS) to generate cytotoxic reactive oxygen species (ROS). The non-selective nature of these ROS helps minimize the risk of resistance development. Despite the development of numerous PSs, limited data on their exact mechanisms and bacterial targets still hinders broader clinical use. The focus of this study is to address this gap by capturing pathway-specific responses to sub-lethal photodynamic stress using a panel of transcriptional biosensors in E. coli K-12 strain MG1655. Our results indicate that methylene blue (MB) primarily causes oxidative stress in the cytosol while silicon phthalocyanine derivative (SiPc) induces envelope stress at physiological conditions. By monitoring well-characterized stress response pathways, our method offers a valuable tool for elucidating the physiological effects of aPDT and guiding more detailed mechanistic or transcriptomic studies.