Antimicrobial Effect of Reactive Oxygen Species (ROS) Generated from Ultraviolet (UV-A) Light Exposure of Benzoic Acid

Introduction: Benzoic acid (BA) is a commonly used preservative whose activity is based on the intracellular acidification. However, in a unique observation, we found that the exposure of 5 mM BA to UV-A (365 nm) light for 30 minutes caused more than 5±0.77-log reduction in Escherichia coli O157:H7 compared to 0.96±0.26-log reduction in presence of 5 mM BA in dark or 0.39±0.09-log reduction in presence of UV-A alone. We postulate that this synergistic antimicrobial activity of BA and UV-A light was due to photosensitization of BA by UV light that resulted in generation of diverse reactive oxygen species. 

Purpose: To test this hypothesis, we investigated the generation of common ROS such as singlet oxygen and hydrogen peroxide, upon exposure of BA to UV-A (365 nm), B (312 nm) or C (254 nm). 

Methods: Ten mM BA was exposed to UV-A (2,015 μW/cm² intensity), B (1,427.5 μW/cm² intensity) or C (4,762 μW/cm²intensity) light with a same dose level. Singlet oxygen was detected by measuring Furfuryl alcohol (FFA) degradation rate by singlet oxygen using high pressure liquid chromatography (HPLC). Hydrogen peroxide generation was measured using ferrous oxidation-xylenol orange (FOX) assay. 

Results: Altogether, 3.57±0.50 pM, 31.74±0.94 pM and 58.66±2.70 pM singlet oxygen and 0.06±0.01 uM, 1.69±0.10 uM and 1.35±0.06 μM hydrogen peroxide were generated from exposure of BA to UV-A, B, and C light, respectively. Thus UV exposure of BA produced diverse ROS and UVC and UVB were more efficient wavelengths than UVA in terms of ROS generation.

Significance: We envision that this synergistic interaction of BA and UV light can be harnessed for diverse applications such as advanced oxidative processes for degradation of xenobiotic compounds and inactivation of pathogenic microorganisms in foods and on food-contact surfaces.