Escherichia coli O157:H7 multiplication in the latex of various lettuce genotypes is negatively correlated with plant peroxidase activity

Authors: GEORGE, ANDREE - Former ARS Employee; Simko, Ivan; Brandl, Maria

Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/4/2025
Publication Date: 2/4/2025
Citation: George, A.S., Simko, I., Brandl, M. 2025. Escherichia coli O157:H7 multiplication in the latex of various lettuce genotypes is negatively correlated with plant peroxidase activity. International Journal of Food Microbiology. 431. Article 111095. https://doi.org/10.1016/j.ijfoodmicro.2025.111095.
DOI: https://doi.org/10.1016/j.ijfoodmicro.2025.111095

Interpretive Summary: Foodborne illness linked to fruit and vegetables poses a great challenge to public health and the agriculture industry. Lettuce has been associated with recurrent outbreaks of E. coli O157:H7. Lettuce is one of few edible plants that produce latex. During lettuce harvest, this latex leaks onto the cut stems and adheres to other lettuce heads, harvesting tools, and packaging. In this study, we screened lettuce cultivars for E. coli O157:H7 multiplication in their latex to identify genotypes that inhibit the pathogen, and further investigated the factors involved in this inhibition. E. coli densities increased on the cut stems covered with latex of all tested cultivars except in breeding line SM09B, a romaine type developed at the USDA, ARS, Salinas. Concentration of sugars, which may promote bacterial growth, and of phenolic compounds and hydrogen peroxide, which generally act as plant defenses against microbes, were quantified in the latex and shown to vary significantly between the cultivars. The activity of peroxidase, an enzyme involved in plant defense to injury was also measured in the latex. Statistical analysis revealed that only peroxidase activity, but neither sugar nor phenolic or hydrogen peroxide quantities, was significantly correlated with E. coli densities in the latex. This negative correlation points to a new trait to investigate for inclusion in lettuce breeding programs in order to inhibit E. coli contaminants in lettuce latex after harvest.

Technical Abstract: Lettuce (Lactuca spp.) is one of few edible plants that produce latex. During lettuce harvest, latex leaks onto the cut stem and adheres to other lettuce heads, harvesting tools, and packaging. Little is known about the colonization of lettuce latex by Shiga toxin-producing E. coli O157:H7 (EcO157), the main causal agent of outbreaks linked to lettuce. We screened 14 lettuce genotypes, including wild lettuce and commercial morphological types, for EcO157 multiplication in their latex-coated cut stems. Change in EcO157 density after its inoculation onto the latex-coated cut stems of these cultivars differed significantly and ranged from a 1.7x decline to a 3.6x increase over 6 h at 25°C; density declined only in one cultivar, a romaine-type lettuce. Latex biochemical properties, such as concentration of sucrose, glucose and fructose, phenolic compounds and H2O2 contents, and peroxidase (POD) activity, were quantified in all cultivars. These traits varied significantly among cultivars, but only POD activity correlated significantly with the change of EcO157 density in the latex (r = -0.553). Total phenolics and H2O2 concentrations were also negatively and significantly correlated (r = -0.608). The inhibitory effect of POD on EcO157 multiplication in lettuce latex may serve as a trait to exploit in order to control microbial contamination. Identification of POD marker loci involved in this inhibition and their integration in lettuce breeding programs may enhance the microbial safety of lettuce.