Suppression of Escherichia coli O157:H7 by Pseudomonas fluorescens A506 on romaine lettuce leaves is independent of iron competition

Authors: XU, TONGZHOU - University Of Georgia; DOGAN, MEHMET - University Of Georgia; MANN, DAVID - University Of Georgia; TISHCHENKO, VIKTOR - University Of Georgia; Brandl, Maria; DENG, XIANGYU - Agriculture University Of Georgia

Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/6/2026
Publication Date: 7/8/2026
Citation: Xu, T., Dogan, M., Mann, D., Tishchenko, V., Brandl, M., Deng, X. 2026. Suppression of Escherichia coli O157:H7 by Pseudomonas fluorescens A506 on romaine lettuce leaves is independent of iron competition. International Journal of Food Microbiology. 460. Article 111955. https://doi.org/10.1016/j.ijfoodmicro.2026.111955.
DOI: https://doi.org/10.1016/j.ijfoodmicro.2026.111955

Interpretive Summary: Lettuce production in the USA is valued at over $4 bi. annually. Contamination of romaine lettuce with Shiga toxin-producing Escherichia coli O157:H7 (EcO157) is an important food safety problem that severely impacts the leafy greens industry economically as well as public health. The biocontrol agent Pseudomonas fluorescens A506 is commercially available and used in tree fruit production, but its efficacy against E. coli on leafy greens is unknown. We demonstrated here that A506 significantly suppresses growth and survival of EcO157 on lettuce leaves. A506 is a high producer of siderophores, allowing this strain to efficiently acquire iron under iron-limiting conditions. Given that iron availability is low on leaf surfaces, we tested whether competition for iron underlies the mechanism by which A506 inhibits EcO157 on lettuce. Neither pre-inoculation with a siderophore-deficient mutant of A506, nor iron supplementation, re-established EcO157 population sizes on the leaves to levels observed in the control treatment. Additionally, EcO157 genes involved in the production of the siderophore enterobactin did not show increased expression during co-colonization of leaves with A506, indicating that EcO157 was not depleted in available iron due to the presence of A506. Overall, these results suggest that iron competition is unlikely the primary driver of A506-mediated inhibition of EcO157 on romaine lettuce and that other yet unknown mechanisms are at play. Understanding the mechanisms mediating inhibition of EcO157 by a biocontrol agent is critical to ensuring its efficacious use. Our findings contribute to devising novel approaches to control EcO157 on lettuce leaves and potentially other foodborne pathogens, and on other fruit and vegetables.

Technical Abstract: Contamination of romaine lettuce with Escherichia coli O157:H7 (EcO157) remains a persistent food safety concern due to the pathogen’s ability to survive on leaf surfaces. Although Pseudomonas fluorescens A506 is well studied as a biocontrol agent in tree fruit production, its ability to suppress EcO157 on leafy greens has not been evaluated. In this study, romaine lettuce leaves were pre-inoculated with A506 and inoculated with EcO157 strain MD41 24 h later. Across all MD41 inoculation levels (3, 4, 5, or 6 log CFU/g lettuce), pre-colonization with A506 significantly reduced MD41 population sizes significantly by 1–2 log CFU/g over 72 h compared with the control. A506 is a high producer of siderophores, allowing this strain to efficiently acquire iron under iron-limiting conditions and comptete with other bacterial species. Cell-free supernatants from A506 grown under iron-limited conditions inhibited MD41 growth in minimal medium. This inhibition was reversed by iron repletion and not observed with the pyoverdine-deficient mutant A506-1, indicating an iron-dependent effect of A506 on MD41 in vitro. In contrast, on romaine lettuce, neither pre-inoculation with A506-1 nor iron supplementation re-established MD41 population sizes to levels observed in the control. Additionally, expression of MD41 entS, entB, and fepA, which are involved in siderophore production and iron acquisition, was not increased during co-colonization of leaves with A506. Overall, these results suggest that iron competition is unlikely the primary driver of A506-mediated inhibition of EcO157 on romaine lettuce and that other yet unknown mechanisms are at play.