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

Effects of ferulic acid combined with light irradiation on deoxynivalenol and its production in Fusarium graminearum

This study aimed to investigate the effects of ferulic acid (FA), a natural phenolic phytochemical, in combination with light irradiation at three wavelengths (365, 385 and 405 nm) on the concentration and toxicity of deoxynivalenol (DON), a mycotoxin produced by Fusarium graminearum. Moreover, this study examined the influence of the combination treatment on DON production in the cultured fungus. FA activated by light at a peak wavelength of 365 nm exhibited the most effective decrease in DON concentration of the tested wavelengths; a residual DON ratio of 0.23 at 24 h exposure was observed, compared with the initial concentration. The reduction in DON using 365-nm light was dependent on the concentration of FA, with a good correlation (r2 = 0.979) between the rate constants of DON decrease and FA concentration, which was confirmed by a pseudo-first-order kinetics analysis of the photoreaction with different FA concentrations (50-400 mg/L) for 3 h. The viability of HepG2 cells increased by 56.7% following in vitro treatment with a mixture containing the photoproducts obtained after treatment with 20 mg/L DON and 200 mg/L FA under 365-nm irradiation for 6 h. These results suggested that the photoreaction of FA under 365-nm irradiation induces the detoxification of DON through degradation or modification of DON. The antifungal effects of the combination (FA and 365-nm light) on F. graminearum were investigated. Conidia treated with the combination did not show additive or synergistic inhibition of fungal biomass and DON production in 7-day cultivated fungal samples compared with samples after single treatment. However, successive treatment, composed of 90 min irradiation at 365 nm and then treatment with 200 mg/L FA for 90 min in the dark, suppressed fungal growth and DON yield to 70% and 25% of the untreated sample level, respectively. This photo-technology involving the two treatment methods of 365-nm irradiation and FA addition as a food-grade phenolic acid in combination or successively, can aid in developing alternative approaches to eliminate fungal contaminants in the fields of environmental water and agriculture. However, further research is required to explore the underlying mechanisms of DON decontamination and its biosynthesis in F. graminearum.

Antimicrobial action of phenolic acids combined with violet 405-nm light for disinfecting pathogenic and spoilage fungi

The aim of this study is to investigate the fungicidal spectrum of six phenolic-cinnamic and -benzoic acid derivatives using four fungi, Aspergillus niger, Cladosporium cladosporioides, Trichophyton mentagrophytes and Candida albicans, in a photocombination system with violet 405-nm light. This is the first study to examine the fungicidal mechanism involving oxidative damage using the conidium of A. niger, as well as an assessment of cellular function and chemical characteristics. The results of the screening assay indicated that ferulic acid (FA) and vanillic acid (VA), which possess 4-hydroxyl and 3-methoxy groups in their phenolic acid structures, produced synergistic activity with 405-nm light irradiation. FA and VA (5.0 mM) significantly decreased the viability of A. niger by 2.4 to 2.6-logs under 90-min irradiation. The synergistic effects were attenuated by the addition of the radical scavenger dimethyl sulfoxide. Generation of reactive oxygen species (ROS), such as hydrogen peroxide and hydroxyl radicals, were confirmed in the phenolic acid solutions tested after irradiation with colorimetric and electron spin resonance analyses. Adsorption of FA and VA to conidia was greater than other tested phenolic acids, and produced 1.55- and 1.85-fold elevation of intracellular ROS levels, as determined using an oxidant-sensitive probe with flow cytometry analysis. However, cell wall or membrane damage was not the main mechanism by which the combination-induced fungal death was mediated. Intracellular ATP was drastically diminished (5% of control levels) following combined treatment with FA and light exposure, even under a condition that produced negligible decreases in viability, thereby resulting in pronounced growth delay. These results suggest that the first stage in the photofungicidal mechanism is oxidative damage to mitochondria or the cellular catabolism system associated with ATP synthesis, which is a result of the photoreaction of phenolic acids adsorbed and internalized by conidia. This photo-technology in combination with food-grade phenolic acids can aid in developing alternative approaches for disinfection of pathogenic and spoilage fungi in the fields of agriculture, food processing and medical care.

Antifungal action of the combination of ferulic acid and ultraviolet-A irradiation against Saccharomyces cerevisiae

AIMS

To examine the antifungal action of photocombination treatment with ferulic acid (FA) and ultraviolet-A (UV-A) light (wavelength, 365 nm) by investigating associated changes in cellular functions of Saccharomyces cerevisiae.

METHODS AND RESULTS

When pre-incubation of yeast cells with FA was extended from 0.5 to 10 min, its photofungicidal activity increased. Flow cytometry analysis of stained live and dead cells revealed that 10-min UV-A exposure combined with FA (1 mg ml^-1 ) induced a ~99.9% decrease in cell viability although maintaining cell membrane integrity when compared with pre-exposure samples. When morphological and biochemical analysis were performed, treated cells exhibited an intact cell surface and oxidative DNA damage similar to control cells. Photocombination treatment induced cellular proteins oxidation, as shown by 2.3-fold increasing in immunostaining levels of ~49-kDa carbonylated proteins compared with pre-irradiation samples. Pyruvate kinase 1 (PK1) was identified by proteomics analysis as a candidate protein whose levels was affected by photocombination treatment. Moreover, intracellular ATP levels decreased following FA treatment both in darkness and with UV-A irradiation, thus suggesting a possible FA-induced delay in cell growth.

CONCLUSIONS

FA functions within the cytoplasmic membrane; addition of UV-A exposure induces increased oxidative modifications of cytosolic proteins such as PK1, which functions in ATP generation, without causing detectable genotoxicity, thus triggering inactivation of yeast cells.

SIGNIFICANCE AND IMPACT OF THE STUDY

Microbial contamination is a serious problem that diminishes the quality of fruits and vegetables. Combining light exposure with food-grade phenolic acids such as FA is a promising disinfection technology for applications in agriculture and food processing. However, the mode of photofungicidal action of FA with UV-A light remains unclear. This study is the first to elucidate the mechanism using S. cerevisiae. Moreover, proteomics analyses identified a specific cytosolic protein, PK1, which is oxidatively modified by photocombination treatment.

Screening of antimicrobial synergism between phenolic acids derivatives and UV-A light radiation

The objective of this study was to investigate synergistic antibacterial activity based on a combination of UV-A light and three classes of food grade compounds: benzoic acid derivatives, cinnamic acid derivatives, and gallates. By using Escherichia coli O157:H7 as the model strain, it was observed that three cinnamic acid derivatives (ferulic acid, coumaric acid, and caffeic acid) and one benzoic acid derivative (2,5-dihydroxybenzoic acid) presented strong synergistic antibacterial activity with UV-A light radiation, where 1 mM levels of these compounds plus with 15 min of UV-A light (total light dose of 6.1 cm^-2) led to more than 7-log CFU mL^-1 of bacterial inactivation. In contrast, synergistic antibacterial activity between UV-A light and most benzoic acid derivatives (benzoic acid, gallic acid, vanillic acid, and 2,5-dimethoxybenzoic acid) were only observed after higher concentrations of these compounds were applied (10 mM). Lastly, from the three gallates tested (methyl gallate, ethyl gallate, and propyl gallate), only propyl gallate showed strong antibacterial synergism with UV-A light, where 10 mM of propyl gallate plus 15 min of UV-A light led to approximately 6.5-log of bacterial reduction. Presence of antioxidant compounds mitigated the light-mediated antibacterial activity of gallic acid, 2,5-dihydroxybenzoic acid, and propyl gallate. Similarly, the light-mediated antibacterial activity of these compounds was significantly (P < 0.05) reduced against metabolic-inhibited bacterial cells (sodium azide pretreatment). On the other hand, the antibacterial synergism between ferulic acid and UV-A light was not affected by the presence of antioxidants or the metabolic state of the bacterial cells. Due to the increasing concerns of antimicrobial resistant (AMR) pathogens, the study also investigated the proposed synergistic treatment on AMR Salmonella. Combinations of 1 mM of ferulic acid or 1 mM of 2,5-dihydroxybenzoic acid with UV-A light radiation was able to inactivate more than 6-log of a multi-drug resistant Salmonella Typhimurium strain.

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.

Enhanced Antimicrobial Activity Based on a Synergistic Combination of Sublethal Levels of Stresses Induced by UV-A Light and Organic Acids

The reduction of microbial load in food and water systems is critical for their safety and shelf life. Conventionally, physical processes such as heat or light are used for the rapid inactivation of microbes, while natural compounds such as lactic acid may be used as preservatives after the initial physical process. This study demonstrates the enhanced and rapid inactivation of bacteria based on a synergistic combination of sublethal levels of stresses induced by UV-A light and two food-grade organic acids. A reduction of 4.7 ± 0.5 log CFU/ml in Escherichia coli O157:H7 was observed using a synergistic combination of UV-A light, gallic acid (GA), and lactic acid (LA), while the individual treatments and the combination of individual organic acids with UV-A light resulted in a reduction of less than 1 log CFU/ml. Enhanced inactivation of bacteria on the surfaces of lettuce and spinach leaves was also observed based on the synergistic combination. Mechanistic investigations suggested that the treatment with a synergistic combination of GA plus LA plus UV-A (GA+LA+UV-A) resulted in significant increases in membrane permeability and intracellular thiol oxidation and affected the metabolic machinery of E. coli In addition, the antimicrobial activity of the synergistic combination of GA+LA+UV-A was effective only against metabolically active E. coli O157:H7. In summary, this study illustrates the potential of simultaneously using a combination of sublethal concentrations of natural antimicrobials and a low level of physical stress in the form of UV-A light to inactivate bacteria in water and food systems.IMPORTANCE There is a critical unmet need to improve the microbial safety of the food supply, while retaining optimal nutritional and sensory properties of food. Furthermore, there is a need to develop novel technologies that can reduce the impact of food processing operations on energy and water resources. Conventionally, physical processes such as heat and light are used for inactivating microbes in food products, but these processes often significantly reduce the sensory and nutritional properties of food and are highly energy intensive. This study demonstrates that the combination of two natural food-grade antimicrobial agents with a sublethal level of physical stress in the form of UV-A light can greatly increase microbial load inactivation. In addition, this report elucidates the potential mechanisms for this synergistic interaction among physical and chemical stresses. Overall, these results provide a novel approach to develop antimicrobial solutions for food and water systems.

Highly Efficient Synthesis of an Emerging Lipophilic Antioxidant: 2-Ethylhexyl Ferulate

Ferulic acid in ester form has shown a stronger ability in ameliorating certain pathological conditions and inhibiting lipid oxidation. In present study, a solvent-free and reduced pressure evaporation system was developed for lipase-catalyzed synthesis of 2-ethylhexyl ferulate (2-EF) from ferulic acid and 2-ethylhexanol. A Box-Behnken design with response surface methodology (RSM) and artificial neural network (ANN) was selected to model and optimize the process. Based on the yields of 2-EF, reaction temperature was shown to be the most important process factor on the molar conversion among all variables. The residual values and the coefficient of determination (R²) calculated from the design data indicated that ANN was better than RSM in data fitting. Overall, the present lipase-catalyzed approach for 2-EF synthesis at low reaction temperature in a reduced pressure evaporation system shows high 2-EF production efficiency. Notably, this approach can reduce the enzyme denaturation and ferulic acid oxidation that usually occur during long-term biosynthetic operations at high temperature.