Downy mildew disease promotes the colonization of romaine lettuce by Escherichia coli O157:H7 and Salmonella enterica

Authors: Simko, Ivan; Zhou, Yaguang; Brandl, Maria

Submitted to: BMC Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/22/2015
Publication Date: 2/4/2015
Publication URL: http://www.biomedcentral.com/1471-2180/15/19
Citation: Simko, I., Zhou, Y., Brandl, M. 2015. Downy mildew disease promotes the colonization of romaine lettuce by Escherichia coli O157:H7 and Salmonella enterica. BMC Microbiology. 15:19. doi: 10.1186/s12866-015-0360-5.

Interpretive Summary: Downy mildew, a plant disease caused by the fungus Bremia lactucae, is endemic in many lettuce growing regions of the world. Invasion by plant pathogens may create new portals and opportunities for microbial colonization of plants. The occurrence of outbreaks of Escherichia coli O157:H7 (EcO157) and Salmonella enterica infections linked to lettuce prompted us to investigate the role of downy mildew in the colonization of Romaine lettuce by these human pathogens. The population size of both pathogens on lettuce leaves increased 10^5-fold in downy mildew necrotic lesions but only 100-fold on the healthy tissue. Under conditions of desiccation on the leaf surface, 1500- and 500x more EcO157 cells survived in the diseased necrotic tissue 48 hours after inoculation than in the healthy and chlorotic tissue, respectively. Additionally, a downy mildew resistant lettuce line supported significantly fewer EcO157 cells in the diseased tissue that a cultivar that is highly susceptible to the fungal pathogen. Our results indicate that downy mildew significantly alters the behavior of enteric pathogens in the lettuce phyllosphere and that plant genetics may be worthy of exploring to lower the increased risk of microbial contamination caused by this plant disease.

Technical Abstract: Downy mildew, a plant disease caused by the fungus Bremia lactucae, is endemic in many lettuce growing regions of the world. Invasion by plant pathogens may create new portals and opportunities for microbial colonization of plants. The occurrence of outbreaks of Escherichia coli O157:H7 (EcO157) and Salmonella enterica infections linked to lettuce prompted us to investigate the role of downy mildew in the colonization of Romaine lettuce by these human pathogens. Whereas both EcO157 and S. enterica population sizes increased 100-fold on healthy leaf tissue under conditions of warm temperature and free water on the leaves, they increased by 105-fold in necrotic lesions caused by B. lactucae. Confocal microscopy of GFP-EcO157 in the necrotic tissue confirmed its massive population density and association with the fungal hyphae. Multiplication of EcO157 in the diseased tissue was inhibited significantly in the downy mildew-resistant lettuce line RH08-0464 compared with that in the more susceptible cultivar Triple Threat. qRT-PCR quantification of expression of the plant basal immunity gene PR-1 revealed that this gene was highly transcribed in line RH08-0464 but had low activity in cultivar Triple Threat, indicating that it may be one of the factors involved in the differential growth of the human pathogen in B. lactucae lesions between the two accessions. Additionally, downy mildew disease had a significant effect on the colonization of EcO157 at high RH (90-100%) and on its persistence at lower RH (65-75%). The latter conditions, which promoted overall dryness of the lettuce leaf surface, allowed for only 0.0011 and 0.0028% EcO157 cell survival in healthy and chlorotic tissue, respectively, whereas 1.58% of the cells survived in necrotic tissue. Our results indicate that downy mildew significantly alters the behavior of enteric pathogens in the lettuce phyllosphere and that plant genetics may be worthy of exploring to lower the increased risk of microbial contamination caused by this plant disease.