SF6019 - Investigation of Routes for Transfer of Food and Water-borne Pathogens to Produce

Researcher:

Dr. Mansel Griffiths, Dept. of Food Science, University of Guelph

Objectives:

1. To determine if Salmonella, E. coli 0157:H7 and L. monocytogenes can enter the edible portions of lettuce, tomatoes, cucumbers and strawberries through contamination of soil by water or manure.
   

2. To determine if root damage caused by mechanical action or by plant pathogens influences the systemic spread of foodborne pathogens.

Expected Benefits:

To allow identification of risks associated with the presence of pathogens in soil and help identify strategies to control these hazards.

Summary of Research Results:

The growth and persistence of Escherichia coli O157:H7, Salmonella Typhimurium and Listeria monocytogenes on a diverse range of plant types over extended cultivation periods was studied. When introduced on the seed of carrot, cress, lettuce, radish, spinach and tomato all the pathogens became rapidly established shortly after germination, attaining cell densities in the order of (5.5-6.5 log cfu/g). In general, E. coli O157:H7 and L. monocytogenes became established and persisted at significantly higher levels on seedlings (9 days post germination) compared to Salmonella. E. coli O157:H7 became internalized in cress, lettuce, radish and spinach seedlings but was not recovered within the tissues of mature plants. Internalization of Salmonella was also observed in lettuce and radish but not cress or spinach seedlings. In contrast, L. monocytogenes did not internalize into seedlings but did persist on the surface of plants throughout the cultivation period. Co-inoculation of isolates recovered from the rhizosphere of plants did not significantly affect the numbers or persistence of human pathogens. The only exception was with Enterobacter cloacae that reduced E. coli O157:H7 Ph1 and L. monocytogenes levels by ca. 1 log cfu/g on lettuce. The bioluminescent phenotype of E. coli O157:H7 Ph1 strain used demonstrated that the human pathogen became established on the roots of growing plants. Scanning electron micrographs of root seedlings suggested that E. coli O157:H7 Ph1 preferentially colonized the root junctions of seedlings. It is proposed that such colonization sites enhanced the persistence of E. coli O157:H7 on plants and facilitated internalization into the inner tissue of developing seedlings. The results obtained from this study suggest that the risk associated with internalized human pathogens in salad vegetables at harvest is low. Nevertheless, the introduction of human pathogens at an early stage of plant development could enhance persistence in the rhizosphere. The probability of contamination of the spinach and lettuce plants is greater when they are exposed to the pathogens just prior to harvest as compared to exposure at the mid-stage of growth.

Human pathogens introduced through irrigation water (drip irrigation) at the flowering stage of tomato and cucumber plants did not contaminate the subsequent fruits, though they persist on the roots of these plants. However, when E. coli O157:H7, Salmonella and Listeria monocytogenes were introduced onto the flowers of cucumber, tomato and strawberries the subsequent fruits were contaminated.

Mechanical disruption of the lateral root of spinach did not favor the uptake of the E. coli O157:H7 by the plant; on the contrary, the viability of E. coli O157:H7 was diminished around the cut edge. Disruption of the root hair, also, did not favor the uptake of E. coli O157:H7 by the spinach plants. But, removal of the root hairs did not affect the viability or colonization of roots by E. coli O157:H7. Co-inoculation with the phyto-pathogen Pseudomonas syringae DC3000 did not affect the uptake of E. coli O157:H7 or generic E. coli by the spinach plants, and neither did damage to the tomato and spinach roots by nematodes.