Browsing by Author "Melotto, Maeli"

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    Intraspecies competition among Salmonella enterica isolates in the lettuce leaf apoplast
    (2024) Jacob, Cristian; Student, Joseph; Bridges, David F.; Chu, Weiping; Porwollik, Steffen; Mcclelland, Michael; Melotto, Maeli
    Multiple Salmonella enterica serovars and strains have been reported to be able to persist inside the foliar tissue of lettuce (Lactuca sativa L.), potentially resisting washing steps and reaching the consumer. Intraspecies variation of the bacterial pathogen and of the plant host can both significantly affect the outcome of foliar colonization. However, current understanding of the mechanisms underlying this phenomenon is still very limited. In this study, we evaluated the foliar fitness of 14 genetically barcoded S. enterica isolates from 10 different serovars, collected from plant and animal sources. The S. enterica isolates were vacuum-infiltrated individually or in pools into the leaves of three- to four-week-old lettuce plants. To estimate the survival capacity of individual isolates, we enumerated the bacterial populations at 0- and 10- days post-inoculation (DPI) and calculated their net growth. The competition of isolates in the lettuce apoplast was assessed through the determination of the relative abundance change of barcode counts of each isolate within pools during the 10 DPI experimental period. Isolates exhibiting varying apoplast fitness phenotypes were used to evaluate their capacity to grow in metabolites extracted from the lettuce apoplast and to elicit the reactive oxygen species burst immune response. Our study revealed that strains of S. enterica can substantially differ in their ability to survive and compete in a co-inhabited lettuce leaf apoplast. The differential foliar fitness observed among these S. enterica isolates might be explained, in part, by their ability to utilize nutrients available in the apoplast and to evade plant immune responses in this niche.
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    Lettuce immune responses and apoplastic metabolite profile contribute to reduced internal leaf colonization by human bacterial pathogens
    (Springer Nature, 2025) Jacob, Cristián; Melotto, Maeli
    Background Human bacterial pathogens such as Salmonella enterica and Escherichia coli can colonize the apoplast of leafy greens, where they may evade standard sanitization measures and persist until produce consumption. Bacterial survival in this niche is influenced by plant immune responses that may vary according to bacterial species and plant genotypes. The variability in immune responses has been associated with differences in pathogen persistence capacity within the phyllosphere. In addition, emerging evidence suggests that preexisting and inducible plant metabolites contribute to either restricting or facilitating colonization of human pathogens in plant tissues. Identifying the molecular mechanisms underlying these interactions is crucial for developing strategies to mitigate contamination in fresh produce. Results We characterized whole-leaf transcriptome and apoplast metabolome profiles of three lettuce cultivars upon exposure to the human pathogenic bacteria S. enterica ser. Typhimurium 14028s and E. coli O157:H7. The lettuce genotypes Lollo Rossa and Green Towers exhibited stronger transcriptional modulation, primarily associated with defense-related processes and showed reduced bacterial survival in their apoplast. Surprisingly, Green Towers did not generate callose deposition or reactive oxygen species burst responses at levels comparable to that of Lollo Rossa, suggesting it has distinct modifications in the apoplastic conditions that restrict pathogen persistence. Apoplastic metabolomic profiling revealed specific compounds alterations in Green Towers linked to bacterial survival, indicating their potential role in this genotype’s defense mechanism. In contrast, the lettuce cultivar Red Tide exhibited minimal transcriptional and metabolic modulation, lack of robust defense activation, which was accompanied by apoplastic bacterial survival. Conclusions This study provides evidence that lettuce cultivars exhibit distinct molecular responses that may influence the persistence of human bacterial pathogens in the leaf apoplast. The results indicate that both immune response activation and metabolite composition may contribute to restrict apoplastic bacterial persistence or growth. These findings offer novel insights into the genetic and biochemical factors shaping lettuce-pathogen interactions, which might inform breeding programs and agronomic practices aimed at enhancing food safety.