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United States Department of Agriculture

Agricultural Research Service

Research Project: MOLECULAR MICROBIOLOGY AND CONTROL OF ENTERIC PATHOGENS THAT CONTAMINATE FRESH PRODUCE

Location: Produce Safety and Microbiology Research

Title: Plant lesions promote the rapid multiplication of Escherichia coli O157:H7 on post-harvest lettuce

Author
item Brandl, Maria

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 7, 2008
Publication Date: September 1, 2008
Citation: Brandl, M. 2008. Plant lesions promote the rapid multiplication of Escherichia coli O157:H7 on post-harvest lettuce. Applied and Environmental Microbiology. 74(17):5285-5289.

Interpretive Summary: Several outbreaks of Escherichia coli O157:H7 (EcO157) infections have been associated with minimally processed leafy vegetables in the U.S. Harvesting and processing cause plant tissue damage. In order to assess the role of plant tissue damage in the contamination of leafy greens with EcO157, the effect of mechanical, physiological, and plant disease lesions on the growth of this pathogen on post-harvest Romaine lettuce was investigated. Within only four hours after inoculation, the population sizes of EcO157 increased 4.0-, 4.5-, and 11.0-fold on lettuce leaves that were mechanically bruised, cut into large pieces, or shredded into multiple pieces, respectively. During the same time, EcO157 population sizes increased only 2-fold on leaves that were left intact after harvest. Also, the population size of EcO157 was 27 times greater on young leaves affected by soft rot due to infection by Erwinia chrysanthemi, than on healthy middle leaves. Confocal microscopy revealed that leaf tip burn lesions, which are caused by a common physiological disorder of lettuce, harbored dense populations of EcO157 cells both internally and externally. Investigation of the colonization of cut lettuce stems by EcO157 showed that the pathogen grew 11-fold over four hours of incubation after its inoculation onto the stems, from which large amounts of latex were released. The results of this study indicate that plant tissue damage of various types can promote significant multiplication of EcO157 over a short time, and suggest that harvesting and processing are critical control points in the prevention or reduction of EcO157 contamination of lettuce.

Technical Abstract: Several outbreaks of Escherichia coli O157:H7 (EcO157) infections have been associated with minimally processed leafy vegetables in the U.S. Harvesting and processing cause plant tissue damage. In order to assess the role of plant tissue damage in the contamination of leafy greens with EcO157, the effect of mechanical, physiological, and plant disease lesions on the growth of this pathogen on post-harvest Romaine lettuce was investigated. Within only four hours after inoculation, the population sizes of EcO157 increased 4.0-, 4.5-, and 11.0-fold on lettuce leaves that were mechanically bruised, cut into large pieces, or shredded into multiple pieces, respectively. During the same time, EcO157 population sizes increased only 2-fold on leaves that were left intact after harvest. Also, the population size of EcO157 was 27 times greater on young leaves affected by soft rot due to infection by Erwinia chrysanthemi, than on healthy middle leaves. Confocal microscopy revealed that leaf tip burn lesions, which are caused by a common physiological disorder of lettuce, harbored dense populations of EcO157 cells both internally and externally. Investigation of the colonization of cut lettuce stems by EcO157 showed that the pathogen grew 11-fold over four hours of incubation after its inoculation onto the stems, from which large amounts of latex were released. The results of this study indicate that plant tissue damage of various types can promote significant multiplication of EcO157 over a short time, and suggest that harvesting and processing are critical control points in the prevention or reduction of EcO157 contamination of lettuce.

Last Modified: 9/22/2014
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