Bactericidal Action of Ethanol-soluble Lignin from Steam-exploded Rice Straw Combined with Ultraviolet-A Irradiation

This study aimed to investigate the bactericidal effect of ethanol-soluble lignin (ESL) from rice straw under ultraviolet-A (UV-A) irradiation. Following steam explosion pretreatment of rice straw, ethanol extraction was used to separate the lignin. The total phenolic content (TPC) in the ESL was 258 mg/g (gallic acid equivalent). Under UV-A irradiation, ESL exhibited bactericidal activity against Gram-positive and -negative bacteria at ≤10 mg TPC/L. Fluorescence experiments demonstrated significant adsorption of ESL to the cell surface of Staphylococcus aureus. Scanning electron microscopy revealed membrane deformation, producing many vesicles on the cell surface, when the ESL was activated by UV-A irradiation. These morphological changes were associated with an increase in membrane permeability, which elevated cell counts stained with propidium iodide to 51% and resulted in a 57% decrease in the intracellular ATP level. ESL exposed to UV-A light generated hydrogen peroxide. Addition of a scavenger of reactive oxygen species (ROS) or replacement of dissolved oxygen with argon gas in a bacterial suspension significantly suppressed the synergistic bactericidal activity of ESL and UV-A radiation, indicating that ROS are involved in the bactericidal mechanism. Organosolv lignin pretreated using steam explosion can be developed for antimicrobial application in combination with UV-A irradiation.