Downy mildew disease promotes the colonization of romaine lettuce by bacterial pathogens

Infection by plant pathogens may create new ways for microbial colonization of plants. Biotrophic and necrotrophic fungi responsible for post-harvest decay have been shown in several studies to enhance the colonization of produce by enteric pathogens.

Glomerella cingulata infection of apple fruit promoted proliferation of Listeria monocytogenes and Escherichia coli O157, Alternaria alternata and Cladosporium spp. had a positive effect on Salmonella enterica colonization of tomato fruit and Fusarium spp. prolonged the survival of E. coli O157 on tomato under storage conditions but not that of L. monocytogenes. These findings suggest that the effect of plant pathogenic fungi on enteric pathogens in plant tissue may vary depending on several factors.

Contamination of lettuce with the human enteric pathogens, E. coli O157:H7 and Salmonella enteric, has caused several outbreaks of foodborne disease all over the world. Downy mildew, a fungal disease of lettuce caused by Bremia lactucae, is endemic to many important lettuce producing areas of California. The occurrence of outbreaks of E. coli O157:H7 and Salmonella enterica Typhimurium infections due to lettuce consumption has prompted USDA scientists to investigate the role of downy mildew in the colonization of romaine lettuce by these human pathogens under controlled laboratory conditions.

Bremia lactucae

For the study, two cultivars and one breeding line of romaine lettuce with different levels of field resistance to B. lactucae were used in the experiments:

susceptible cultivar Triple Threat with a low level of resistance,
cultivar Green Towers with an intermediate level of resistance,
breeding line RH08-0464 with a high level of resistance

Results showed that whereas both E. coli O157 and S. Typhimurium population increased 102-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.

Multiplication of E. coli O157 in the diseased tissue was significantly lower in the RH08-0464 lettuce line, which has a high level of resistance to downy mildew than in the more susceptible cultivar Triple Threat.

qRT-PCR quantification of expression of the plant basal immunity gene PR-1 revealed that this gene had greater transcriptional activity in line RH08-0464 than 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 lettuce accessions.

Additionally, downy mildew disease had a significant effect on the colonization of E. coli O157 at high relative humidity (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% E. coli O157 cell survival in healthy and chlorotic tissue, respectively, whereas 1.58% of the cells survived in necrotic tissue.

Scientists conclude that downy mildew significantly affects the behavior of enteric pathogens in the lettuce phyllosphere and that breeding and developing of new lettuce cultivars selected for B. lactucae resistance may lower the increased risk of microbial contamination caused by this plant pathogen.

Source: Ivan Simko, Yaguang Zhou, Maria T. Brandl, 'Downy mildew disease promotes the colonization of romaine lettuce by Escherichia coli O157:H7 and Salmonella enterica', 2015, BMC Microbiology, 15:19, DOI 10.1186/s12866-015-0360-5. Further info: http://www.biomedcentral.com/1471-2180/15/19

Contacts:
Maria T. Brandl
Produce Safety and Microbiology Research Unit
U.S. Department of Agriculture
ARS-Agricultural Research Service Albany, CA 94563, USA
Email: maria.brandl@ars.usda.gov