Population dynamics of Escherichia coli inoculated by irrigation into the phyllosphere of spinach grown under commercial production conditions

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.