Identification and characterization of lettuce cultivars with high inhibitory activity against the human pathogen Escherichia coli O157:H7: Toward a plant-intrinsic hurdle approach to microbial safety

Authors: George, Andree; Simko, Ivan; Brandl, Maria

Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/27/2024
Publication Date: 2/12/2024
Citation: George, A.S., Simko, I., Brandl, M. 2024. Identification and characterization of lettuce cultivars with high inhibitory activity against the human pathogen Escherichia coli O157:H7: Toward a plant-intrinsic hurdle approach to microbial safety. Postharvest Biology and Technology. 211. Article 112816. https://doi.org/10.1016/j.postharvbio.2024.112816.
DOI: https://doi.org/10.1016/j.postharvbio.2024.112816

Interpretive Summary: Outbreaks of produce-associated foodborne illness continue to pose a threat to human health and a challenge to the produce industry. Plant defenses elicited in response to mechanical injury of leaf tissue may serve as a first hurdle to wound colonization by enteric pathogens. Given known cultivar-dependent differences in lettuce resistance to abiotic and biotic stresses, we hypothesized that once wounded, lettuce cultivars that display indicators of a strong defense response against necrotrophic plant pathogens and insect pests would inhibit survival of EcO157. To investigate this hypothesis, EcO157 survival was compared on lettuce cultivars selected based on their resistance to these pathogens and pests. Lettuce leaves of 31 cultivars were inoculated with EcO157, then shredded or left intact, and stored at 5°C for 48 hrs. The reduction of EcO157 survival on shredded tissue compared to intact tissue was significantly greater on five of the selected cultivars. To determine the mechanism of differential EcO157 inhibition, plant compounds known to possess antimicrobial properties, including total phenolics, reactive oxygen species (ROS), and anthocyanin content were quantified in the lettuce cultivars. Additionally, activity of phenylalanine lyase (PAL), peroxidase (POD), and polyphenol oxidase (PPO), enzymes known to play a role in wound response, were measured. Multivariate analyses were performed to find links between the lettuce traits and EcO157 survival. These analyses determined that phenolic content, PAL activity, anthocyanins, and POD are key indicators of EcO157 decline. Conversely, high PPO activity was linked to poor inhibition of EcO157. Our results provide evidence that the production of compounds that inhibit human pathogens may be a useful consideration in the development of new cultivars to enhance the microbial safety of processed lettuce.

Technical Abstract: Foodborne illness linked to fruit and vegetables poses a considerable challenge to horticulture. Processed lettuce has been implicated in recurrent outbreaks of Shiga toxin-producing E. coli (STEC) infection. Plant defenses elicited by mechanical injury may affect STEC inhibition in cut leaves. Intact and cut leaves of 31 lettuce cultivars (mostly Lactuca sativa) selected from 502 cultivars based on resistance to herbivorous insects and phytopathogens were assessed for STEC serovar O157:H7 (EcO157) survival. Total leaf phenolics, anthocyanins, and reactive oxygen species (ROS); and phenylalanine lyase (PAL), peroxidase (POD), and polyphenol oxidase (PPO) activity were quantified in each cultivar. PCA and K-means cluster analysis were applied to group phenotypes and EcO157 population decline. Five cultivars with high resistance to three insects, three phytopathogens, and tip burn also effected significantly greater EcO157 inhibition on cut leaves than other cultivars. High PAL activity, and phenolic and anthocyanin content (PPA); and high PAL and POD activity, were associated with low EcO157 survival. One cultivar with low resistance and defense trait levels showed high EcO157 inhibition, suggesting additional plant traits at play. Basal plant defense traits may inform about enteric pathogen survival on processed lettuce and represent potential intrinsic hurdles to exploit for enhanced microbial safety.