The current situation of fungal, bacterial, and toxin contamination in agriculture and food industries, as well as the emergence of resistant strains of fungi and bacteria caused by frequent and improper application of traditional antimicrobial agents and excessive pesticide residues, has sparked widespread public concern about food safety and environmental risks, prompting researchers to look for efficient antimicrobial substances derived from natural resources to replace or reduce the amount of traditional antimicrobial agents. Polyphenols are a type of secondary metabolite found primarily in the roots, bark, stems, leaves, and fruits of many edible crops and herbs. As the second largest group of chemicals in the plant kingdom, their antimicrobial and antioxidant properties make them a hot research topic in the fields of antimicrobial agent development and food storage. It is critical to evaluate the antimicrobial activity and study the mechanism of action of plant polyphenols in order to promote the development of phenolic compounds for crop disease prevention and control and food preservation industries. In this study, we studied the anti-phytopathogenic fungal, bacterial, and food-borne bacterial activities of various plant polyphenols and their derivatives, as well as the mechanisms of action those polyphenols and their derivatives.
The following are the key findings.
1. Antimicrobial activity screening of plant polyphenols and their derivatives
In this chapter, the antimicrobial activity of phenolic compounds, including flavonoids, phenolic acids, lignans, stilbenes, and simple phenolics, was determined through activity screening. Among them, butyl gallate, isoamyl gallate, trans-ferulic acid, ethyl caffeate, salicylic acid, cinnamic acid, glabridin, pterostilbene, 3'-hydroxypterostilbene, magnolol, honokiol, eugenol, isoeugenol methyl ether, isoeugenol, α-asarone, olivetol, thymol, carvacrol, and a variety of simple alkyl substituted phenols had good anti-phytopathogenic fungal activity. Among them, glabridin, pterostilbene, magnolol, honokiol, thymol, carvacrol and 6-tert-Butyl-m-
cresol had excellent and broad-spectrum antifungal activity against Aspergillus flavus, Penicillium expansum, Rhizopus stolonifer, Monilinia fructicola, and Botrytis cinerea, the EC50 values of five phytopathogenic fungi were all less than 50 μg/mL. Nobiletin had specific antifungal activity against Rhizopus stolonifer, with an EC50 of 14.03 μg/mL. Xanthohumol and isoxanthohumol showed specific antifungal activity against Botrytis cinerea with EC50 of 2.51 and 4.29 μg/mL, respectively. Following that, it was discovered that gallic acid isoamyl ester, magnolol, honokiol and olivetol exerted the best antibacterial activity against four food-borne bacteria, with MIC90 less than 200 μg/mL.
2. Evaluation of antimicrobial activity of stilbenes, study on the mechanism of action and toxin metabolism of Aspergillus flavus
In this chapter, seven stilbene compounds, namely resveratrol, oxyresveratrol, piceatannol, pterostilbene, 3'-hydroxypterostilbene, polydatin and tapinarof, were selected for antimicrobial activity assay and it was found that pterostilbene, 3-hydroxypterostilbene and tapinarof exhibited broad-spectrum antifungal activity against plant pathogenic fungi such as Aspergillus niger, Aspergillus flavus, Penicillium
expansum, Fusarium graminearum, Monilinia fructicola, Botrytis cinerea and Colletotrichum gloeosporioides. Further in vitro activity evaluation showed that pterostilbene exerted the best inhibitory activity against Aspergillus flavus with an EC50
of 15.94 μg/mL for mycelial growth inhibition and 3.86 μg/mL for spore germination inhibition, and concentration-dependent inhibition of Aspergillus flavus mycelial biomass production under liquid culture conditions. In vivo efficacy evaluation revealed that treatment with pterostilbene at 250 and 500 μg/mL could effectively inhibit Aspergillus flavus infestation on peanut. Toxin metabolism studies revealed that 75 μg/mL of pterostilbene significantly inhibited the expression of afl A, afl B, afl C, afl R, afl S and the global regulatory genes of fungal metabolism Lea A and V eA in the toxin metabolism pathway, thereby reducing the biosynthesis of aflatoxin B1 by 37.62%. The results of the mechanism investigation indicated that pterostilbene can induce a large
production of reactive oxygen species in Aspergillus flavus cells, interfere with the intracellular redox balance, cause the collapse of mitochondrial membrane potential, and loss of cell membrane integrity, and eventually lead to the death of Aspergillus flavus cells through various modes of action.
3. Synthesis, evaluation of antimicrobial activity and mechanism of action of acylated phloroglucinol derivatives
In this chapter, the synthesis, antimicrobial activity evaluation and mechanism of action of acylated phloroglucinol derivatives were investigated, and it was found that compound 2b could effectively inhibit the mycelial growth of Botrytis cinerea, Monilinia fructicola and Rhizopus stolonifer with EC50 of 1.39, 1.18 and 7.76 μg/mL, respectively, and had low cytotoxicity and phytotoxicity. In an in vivo antimicrobial
assay, the therapeutic efficiency of compound 2b at 200 μg/mL against gray mold and brown rot was 76.26% and 83.35%, respectively, which was slightly lower than that of the commercial fungicides pyrimethanil and thiophanate-methyl. Preliminary mechanistic studies suggest that compound 2b may exert its antifungal inhibitory effects by inhibiting spore germination, disrupting mycelial cell membrane integrity,
and causing leakage of cell contents.