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Title: Commensal effect of pectate lyases secreted from Dickeya dadantii on the proliferation of Escherichia coli O157:H7 on lettuce leaves

item YAMAZAKI, AKIHIRO - University Of Wisconsin
item LI, JIN - University Of Wisconsin
item HUTCHINS, WILLIAM - University Of Wisconsin
item WANG, LIXIA - University Of Wisconsin
item MA, JINCAI - University Of California
item Ibekwe, Abasiofiok - Mark
item YANG, CHING-HONG - University Of Wisconsin

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/31/2010
Publication Date: 1/1/2011
Citation: Yamazaki, A., Li, J., Hutchins, W.C., Wang, L., Ma, J., Ibekwe, A.M., Yang, C. 2011. Commensal effect of pectate lyases secreted from Dickeya dadantii on the proliferation of Escherichia coli O157:H7 on lettuce leaves. Applied and Environmental Microbiology. 77(1):156-162.

Interpretive Summary: E. coli O157:H7 are known to be associated with severe human diseases while Dickeya dadantii 3937 is a causative agent of soft-rot disease in many economically important crops. In recent years, a great majority of E. coli O157:H7 outbreaks have been traced to consumption of contaminated meat/ leafy greens and healthy cattle are the natural reservoir of E. coli O157:H7. In this study, E. coli O157:H7 and D. dadantii 3937 wild types were co-inoculated on lettuce leaves and measurement of bacterial populations on inoculated leaves were done as well as green fluorescence intensity using flow cytometry. Our results showed that Dickeya dadantii 3937 promotes leaf surface maceration in lettuce for E. coli O157:H7 to attach to the leaf for nutrients and water that resulted in the proliferation of E. coli O157:H7 on lettuce leaves. The results of this comprehensive research will be used by growers and food safety experts to develop improved management and post-harvest food handling strategies for leafy green production.

Technical Abstract: The outbreaks of enterohemorrhagic Escherichia coli O157:H7 from leafy greens are serious food-safety concerns at the present period. Several phytopathogens have been suggested to help persistence and proliferation of the human enteropathogens in phyllosphere. In this work, influence of virulence mechanisms of Dickeya dadantii 3937, a broad-host-range phytopathogen, on the proliferation of human pathogen E. coli O157:H7 EDL933 (EDL933) in the postharvest lettuce phyllosphere was examined by co-inoculation of EDL933 with D. dadantii 3937 derivatives that have mutations in virulence-related genes. A type II secretion system (T2SS)-deficient mutant of D. dadantii 3937, A1919, lost the capability to promote EDL933 multiplication, whereas Ech159, a stress-responsive sigma factor RpoS-deficient mutant, strengthened EDL933 proliferation on lettuce leaves. A spectrophotomeric enzyme activity assay revealed that A1919 completely lost its ability to secrete pectate lyases (Pels), which play a major role in plant tissue maceration. In contrast with A1919, Ech159 showed more than two-fold greater Pel activity compared to wild-type D. dadantii 3937. Increased transcriptional activity of pelD (encodes an endo-pectate lyase) was observed in Ech159 in planta. These results indicate that the pectinolytic activity of D. dadantii 3937 is the dominant determinant of EDL933 proliferation in the lettuce phyllosphere. In addition, RpoS, the general stress response sigma factor involved in cell survival in suboptimal conditions, plays a role in EDL933 proliferation by controlling the production of pectate lyases in D. dadantii 3937.