Determining mode of action and optimum use of phytochemicals as food preservatives

Sweet, Ryan (2022) Determining mode of action and optimum use of phytochemicals as food preservatives. Doctoral thesis, University of East Anglia.

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Abstract

Antimicrobial resistance and the lack of novel antimicrobials are urgent global issues. New compounds with direct antimicrobial action or synergy with current antimicrobials are sorely needed. This thesis attempts to define the potential and mode of action (MOA) of one alternative, novel antimicrobial compound source: phytochemicals. These structurally diverse compounds may be suitable solutions to the above issues. There is a large but contradictory evidence base for the antimicrobial activity of many phytochemicals within the literature. I compared a phytochemical panel using a standardised, OD-based assay to identify antimicrobial activity, followed by quantitative assays, against four foodborne pathogens. From this data, caffeic acid, thymol and a commercial phytochemical preservative substitute (the Prosur NATPRE T-10+ mix) were identified as holding antimicrobial promise. Mutant selection revealed thymol could select for tolerant mutants of Salmonella enterica, Staphylococcus aureus and Pseudomonas aeruginosa at a rate akin to classic antibiotics. Genome sequencing of the mutants and a thymol-challenged Salmonella enterica TraDIS-Xpress library revealed SNPs and key loci for thymol resistance to be heavily associated with efflux and other envelope functions. Transmission electron microscopy of a thymol-selected Salmonella enterica mutant and its parental strain revealed a damaged morphology of the parental strain under thymol exposure, while the mutant possessed a more rounded morphology under control conditions and was unaffected by thymol challenge. This evidence brings support for an envelope-targeting primary MOA for the inhibitory nature of thymol. A food challenge test was undertaken to determine the in situ efficacy of thymol within a vegetarian burger model, where it exerted an antimicrobial effect against inoculated Salmonella enterica. Together this work shows phytochemicals can have potent antimicrobial activity with potential for application as preservatives. The cell membrane seems to be a key target for phytochemicals but resistance due to efflux may be an issue for their wider usage.

Item Type:	Thesis (Doctoral)
Faculty \ School:	Faculty of Science > School of Biological Sciences
Depositing User:	Chris White
Date Deposited:	30 Jan 2023 12:43
Last Modified:	30 Jan 2023 12:43
URI:	https://ueaeprints.uea.ac.uk/id/eprint/90859
DOI:

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