Bactericidal action of ferulic acid with ultraviolet-A light irradiation

The bactericidal activity of ferulic acid (FA) against various microorganisms was remarkably enhanced by ultraviolet-A (UV-A) irradiation (wavelength, 365 nm). However, the bactericidal mechanism in the photo-combination system has not been evaluated. In the present study, this combined treatment was characterized by investigating associated changes in cellular functions of Escherichia coli, including assessments of respiratory activity, lipid peroxidation, membrane permeability, and damage to DNA and the cell surface. FA adsorbed onto and was incorporated into bacterial membranes, and the affinity resulted in decreased respiratory activity and enhanced lipid peroxidation in the cytoplasmic membrane with low-fluence (1.0 J/cm²) UV-A irradiation. Flow cytometry analysis revealed that additional exposure (8 J/cm²) combined with FA (1 mg/mL) induced increased cell permeability, yielding a 4.8-log decrease in the viable cell count. Morphologically, the treated cells exhibited a bacterial membrane dysfunction, producing many vesicles on the cell surface. However, despite this effect on the cell surface, plasmid DNA transformed into FA-treated E. coli maintained supercoiled integrity with negligible DNA oxidation. Our data strongly suggested that FA functions inside and outside the bacterial membrane; UV-A exposure in the presence of FA then causes increased oxidative modification and subsequent disruption of the bacterial membrane, without causing detectable genotoxicity.