Rapid killing of bacteria by a new type of photosensitizer

Photodynamic antimicrobial chemotherapy (PACT) uses non-traditional mechanisms (free radicals) and is a highly advocated method with promise of inactivating drug-resistance bacteria for local infections. However, there is no related drug used in clinical practice yet. Therefore, new photosensitizers for PACT are under active development. Here, we report the synthesis of a series of photosensitizers with variable positive charges (ZnPc(TAP)₄ⁿ⁺, n = 0, 4, 8, 12) and their inactivation against Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria. The binding kinetics of ZnPc(TAP)₄ⁿ⁺ to bacteria were measured by flow cytometer. Reactive oxygen species (ROS) generation mechanism of the photosensitizers was studied. The toxicity of these compounds to human blood cells was also evaluated. These compounds showed negligible toxicity against human erythocytes but potent bactericidal effects. The compound with 8 positive charges, ZnPc(TAP)₄⁸⁺, turned out to have the strongest antibacterial effect among this series of compounds, giving IC₅₀ value of 59 nM at a light dosage of 5 J/cm² toward E. coli. For a multi-resistant E. coli strain, ZnPc(TAP)₄⁸⁺ decreased the bacteria load by 1000-fold at a concentration of 1 μM. Interestingly, ZnPc(TAP)₄¹²⁺, instead of ZnPc(TAP)₄⁸⁺, exhibited the highest amount of binding to bacteria. Flow cytometry studies showed that all PSs have fast binding onto bacteria, reaching saturated binding within 5 min. Mechanistically, ZnPc(TAP)₄¹²⁺ generated ROS primarily via Type I mechanism, while ZnPc(TAP)₄⁴⁺ or ZnPc(TAP)₄⁸⁺ created ROS by both type I and type II mechanisms. ZnPc(TAP)₄ⁿ⁺ are highly potent, rapid-acting and non-toxic photosensitizers capable of inactivating bacteria.