Recent outbreaks of food-borne illnesses associated with the consumption of fresh produce have increased attention on irrigation water as a potential source of pathogen contamination. A better understanding of the behaviour of enteric pathogens introduced into agricultural systems during irrigation will aid in risk assessments and support the development of appropriate farm-level water management practices. For this reason, the survival dynamics of two nalidixic acid resistant strains of Escherichia coli after their spray inoculation into the phyllosphere and soil of field spinach were examined over two growing seasons. E. coli strains NAR, an environmental isolate, and DM3n, a non-pathogenic serotype O157:H7, were applied at rates of 104 to 107cfu/100ml to the fully developed spinach plants that arose subsequent to the harvesting of their upper leafy portions for commercial purposes (secondary-growth plants). After 72h, E. coli on spinach were reduced by 3-5 logs. Culturable E. coli were recovered from plants up to 6days post-inoculation. Survival in soil was greater than in the phyllosphere. Under ambient conditions, the mean 72h first order decay constant computed by Chick's Law was 0.1h-1. Although light reduction studies indicated UV irradiation negatively influenced the persistence of E. coli, a simple relationship between UV exposure and phyllosphere E. coli densities could not be established. E. coli introduced to the leafy portions of spinach via spray irrigation displayed rapid declines in their culturability under the open environmental conditions experienced during this study. A 6day period between the last irrigation and harvest would minimize the risks of E. coli survival in the spinach phyllosphere. E. coli NAR was identified as a possible surrogate for the O157:H7 strain, DM3n.
Bibliographical noteFunding Information:
Funding for this project was provided by the Canada-Nova Scotia Water Supply Expansion Program, and the Canada Research Chairs Program. The contributions of the Nova Scotia Department of Agriculture, Agriculture Agri-Food Canada Research Branch, Melvin Farms Inc. and the Nova Scotia Federation of Agriculture are greatly appreciated. The authors gratefully acknowledge the assistance provided by the Government of Canada Federal Summer Work Experience Program participants, Greg Potter and Sara Dickson.
- E. coli
- UV radiation