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theses

ETD doctoral

Foodborne pathogens are one of the major causes for food spoilage which happens in raw meat, vegetable, and dairy products. The emerging antibiotic resistance acquired by common pathogens is bringing urgency for food industry to explore more options for effective and safe antimicrobial agents. As one of the largest groups of plant metabolites, flavonoids were discovered to possess antimicrobial activity but much of the detailed mechanisms remain unclear.

In this study, we established a QSAR model using published datasets that can summarize and predict the redox behaviors of flavonoids with given structures. Then several flavonoids with similar backbone structures yet varied hyperactive groups were tested for antibacterial activity. We observed that flavonoid antibacterial activity has a positive correlation with its antioxidant potential, which was typically measured in acidic environments. Our ORAC analysis revealed that the most theoretically antioxidant flavonoids may exhibit prooxidant properties due to rapid degradation in weak alkaline conditions, a phenomenon directly associated with their antibacterial activities. We then focused our study on myricetin, the most potent one in the group that contains a C2=C3 double bond and B-ring pyrogallol group structure. We discovered that myricetin undergoes autoxidation to generate reactive oxygen species (ROS) at physiological conditions. We then supplemented enzymes or cofactor metals that are essential for various cellular oxidative pathways to the growth medium of E.coli or L. monocytogenes with myricetin, and noticed significantly corresponding interference or improvement of the inhibitory effect. These data suggested that the antibacterial activities of myricetin are directly mediated by the ROS generated by its autoxidation. We also showed that the ROS generation in turn altered the physiological profile of E.coli cells that may lead to its demise. These alterations include the compromise of membrane integrity, increased leakage of cellular components, as well as transcriptional upregulations of multiple genes associated with oxidative damage prevention, DNA replication or DNA damage repair. Altogether, our findings highlighted the use of the previously underexplored natural flavonoid compound myricetin, among other flavonoids, as a potential and potent antibiotic with elucidated mechanisms of action that could be partially inferred from its structural features. The emergence of such broad-spectrum, strong and natural antibiotics will serve as one of the key solutions to the alarmingly rising problems of antibiotic resistance and will therefore be unmeasurably beneficiary to the food and drug industry.

ETD_11200

Ph.D.

Includes bibliographical references

doi:10.7282/t3-0666-wf04

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