Synergistic Photobactericidal Activity Based on Ultraviolet-A Irradiation and Ferulic Acid Derivatives

Ultraviolet-A (UV-A)-mediated bactericidal activity was enhanced by combined treatment with trans-ferulic acid (trans-FA, compound 1) or its derivatives. Derivative compounds 4 and 10 contain a phenyl group or an l-tyrosine HCl tert-butyl ester, respectively, linked to the carboxyl group of trans-FA. Of the three compounds, 10 exhibited the highest synergistic activity in a photobactericidal assay based on treating Escherichia coli with a derivative compound and UV-A irradiation (wavelength 350-385 nm). Inactivation of viable cells at a 4.9 J cm(-2) UV-A fluence increased from 1.90 to 5.19 logs in the presence of 10 (100 μm); a 4.95-log inactivation was achieved with 10 (5 μm) and a 7.4 J cm(-2) UV-A fluence. Addition of antioxidants significantly suppressed photosynergistic bactericidal activity, suggesting that reactive oxygen species (ROS) are involved in the combined bactericidal mechanism. Flow cytometry revealed that combined treatment with UV-A and compound 10, which showed the highest photobactericidal activity, generates an excess of oxidative radicals in bacterial cells. The bactericidal activity of compound 10 may be due to electrostatic interaction between the molecule's cationic moiety and the cell surface, followed by amplification of ROS generation in the cells.