Transition metal complexes: next-generation photosensitizers for combating Gram-positive bacteria

The rise of antibiotic-resistant Gram-positive bacterial infections poses a significant threat to public health, necessitating the exploration of alternative therapeutic strategies. A photosensitizer (PS) can convert energy from absorbed photon into reactive oxygen species (ROS) for damaging bacteria. This photoinactivation action bypassing conventional antibiotic mechanism is less prone to resistance development, making antibacterial photodynamic therapy (aPDT) highly efficient in combating Gram-positive bacteria. Photodynamic transition metal complexes leveraging the unique properties of metals to enhance the aPDT activity are the next-generation PS. This review provides an overview of metal-based PS for combating Gram-positive bacteria. Based on the structures, these metal-PS could be mainly classified as metal-tetrapyrrole derivatives, ruthenium complexes, iridium complexes, and zinc complexes. PS based on complexes of other transition metals such as silver, cobalt, and rhenium are also presented. Finally, we summarize the advantages and shortcomings of these metal- PS, conclude some critical aspects impacting their aPDT performances and give a perspective on their future development.