Transcriptomic response of Escherichia coli O157:H7 on Romaine lettuce from harvest to storage during the pre-processing interval

Authors: DING, QIAO - University Of Maryland; Gu, Ganyu; Luo, Yaguang; Nou, Xiangwu; MICALLEF, SHIRLEY - University Of Maryland

Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2025
Publication Date: 4/23/2025
Citation: Ding, Q., Gu, G., Luo, Y., Nou, X., Micallef, S. 2025. Transcriptomic response of Escherichia coli O157:H7 on Romaine lettuce from harvest to storage during the pre-processing interval. Postharvest Biology and Technology. https://doi.org/10.1016/j.postharvbio.2025.113594.
DOI: https://doi.org/10.1016/j.postharvbio.2025.113594

Interpretive Summary: A large proportion of leafy greens grown in California and Arizona is transported long distance to locations like the east coast for processing and marketing, a practice known as "forward processing". In recent years, reoccurring E. coli O157:H7 outbreaks associated with romaine lettuce have led to concerns that the long post-harvest delay in romaine lettuce processing due to the need for long distance transportation might play a role in these outbreaks. CDC and FDA have recently characterized the E. coli O157:H7 strains associated with these outbreaks as "reoccurring, emerging, and persistent (REP)". In this study, ARS scientists collaborated with scientists at University of Maryland in examining the global gene expression (transcriptomic analyses) for different E. coli O157:H7 strains under conditions simulating romaine lettuce growth, transportation, and storage. These analyses showed that genes in multiple functional groups, including cell motility and stress responses, were differentially expressed in the REP and the control strains under various conditions simulating the transportation and storage. However, for individual strains, the global gene expression profiles were not significantly affected by the conditions simulating forward processing. The information from this study is useful for better understanding the survival and persistence of the REP strain in the environments of leafy green growth and processing.

Technical Abstract: Commercially harvested lettuce generally undergoes short- or long-haul transportation from the growing region to facilities near the destination markets for fresh-cut processing. The interval of time between harvest and processing varies by location of the processing facility. To determine how duration of the pre-processing interval (1 versus 8 days)may affect food safety risk, we investigated how pre-harvest and post-harvest, pre-processing conditions affected the gene expression patterns of lettuce-associated E. coli O157:H7, using the reference strain EDL933 and a recent, recurring lettuce outbreak strain 2705C. Genes belonging to various functional groups were differentially expressed in E. coli O157:H7 in response to the bacteria transitioning to the lettuce phyllosphere, followed by storage conditions post-harvest. Genes linked to oxidative, osmotic and acid stress, the starvation response, cold shock, antimicrobial stress and biofilm formation were up regulated in both strains. Strain-specific responses included up-regulation of motility and chemotaxis genes in EDL933 only, and involvement of more genes related to oxidative and antimicrobial stress responses in 2705C. Moreover, the two strains yielded differential patterns of expressed genes associated with all functions, suggesting that the two strains adopted distinct genetic mechanisms to adapt to the lettuce phyllosphere. The elevated expression levels of osmotic and cold shock genes, and antimicrobial stress genes in both strains were maintained throughout the post-harvest interval. On the other hand, down-regulation of several genes involved in starvation, oxidative and acid stress, and biofilm formation was detected in the outbreak strain 2705C from pre- to post-harvest conditions, while expression levels in EDL933 remained mostly unchanged. By contrast, genes with functions in the cold shock response and antimicrobial stress were up regulated in the post-harvest EDL933 group. In both strains, the expression of yhcN, involved in oxidative and acid stress, bhsA, involved in biofilm formation, and mdtG and marB, involved in efflux of antimicrobial compounds, were further augmented in post-harvest storage. Except for the stationary phase gene mgtS in EDL933, the few genes that were differentially regulated in E. coli O157:H7 associated with lettuce subjected to a prolonged 8-day pre-processing delay, compared to a short 1-day interval, were down-regulated. No genetic response specific to prolonged storage or fluctuations in atmospheric pressure in E. coli O157:H7 on forward processed lettuce was detected. Overall, these results suggest that E. coli O157:H7 adaptation to the harsh, nutrient-depleted pre-harvest lettuce surface involves stress adaptations that primed the bacteria for post-harvest conditions in storage, in a strain-dependent manner. Adaptation to lettuce was immediate. Prolonged post-harvest storage did not significantly alter gene expression patterns of E. coli O157:H7 associated with source and forward processed lettuce. Findings provide insights on strain differences in transcriptomic patterns of E. coli O157:H7 associated with lettuce pre- and post-harvest which could translate to variable food safety risk.