ARS | Publication request: Variation in reactive oxygen species concentration among lettuce types and modulation of their production via PPO during E. coli O157:H7 colonization of injured lettuce leaves(Abstract)

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: April 14, 2012
Publication Date: June 16, 2012
Citation: Brandl, M., Hua, S.T., Zhou, Y., Sarreal, S.L. 2012. Variation in reactive oxygen species concentration among lettuce types and modulation of their production via PPO during E. coli O157:H7 colonization of injured lettuce leaves(Abstract). Meeting Abstract. [abstract].

Technical Abstract: Minimally processed leafy vegetables are the biggest culprits in produce-linked outbreaks of foodborne illness and E. coli O157:H7 (EcO157) is the predominant causal agent of these epidemics. Harvesting and processing cause plant lesions thus creating new niches for opportunistic colonization of leaves by human pathogens. Due to the low infectious dose of EcO157, identification of bacterial and plant attributes that enable or inhibit its growth on fresh-cut leaves is essential for the development of effective control strategies. Using a lettuce lysate model to mimic the chemical environment resulting from the release of plant cell contents during injury, we observed large differences in the length of the lag phase and in the growth of EcO157 in the lysate of five different lettuce varieties and of different lettuce leaf ages. EcO157 growth rates in the lysates from the various lettuce samples correlated negatively with the concentration of reactive oxygen species (ROS) in the cell lysates (R2=0.5; P<0.001), which was quantified with the Amplex Red assay. On the contrary, EcO157 growth rates were independent of the concentrations of the predominant carbohydrates present in the lysates, as measured by HPLC. Whereas EcO157 growth rates were 10-fold greater in lysates of Iceberg middle leaves compared with those in Romaine middle leaf lysates, they were not different in these types of lettuce when the leaves were injured by shredding. qRT-PCR on lettuce genes that are involved in the plant response to injury showed that the gene coding for PPO, an enzyme that produces ROS and is involved in plant resistance to microbial infection, was repressed during colonization of shredded lettuce by EcO157. Our results suggest that, when colonizing injured leaf tissue, EcO157 may inhibit the production of ROS by modulating the plant defense response and therefore, the outcome of a contamination event.