Abstract
In the field, foodborne pathogens such as enterohemorrhagic Escherichia coli (EHEC) are capable of surviving on produce over time, yet little is known about how these pathogens adapt to this environment. To assess the impact of pre-harvest environmental conditions on EHEC survival, we quantified survival on romaine lettuce under two relative humidity (75% and 45%) and seasonal conditions (March and June). Greenhouse-grown lettuce was spray-inoculated with EHEC and placed in a growth chamber, mimicking conditions typical for June and March in Salinas Valley, California. Bacteria were enumerated on days 0, 1, 3, and 5 post-inoculation. Overall, we found that the effect of relative humidity on EHEC survival depended on the seasonal conditions. Under June seasonal conditions, higher relative humidity led to lower survival, and lower relative humidity led to greater survival, five days post-inoculation. Under March seasonal conditions, the impact of relative humidity on EHEC survival was minimal over the five days. The bacteria were also tested for their ability to survive a chlorine decontamination wash. Inoculated lettuce was incubated under the June 75% relative humidity conditions and then washed with a 50 ppm sodium hypochlorite solution (40 ppm free chlorine). When incubated under June seasonal conditions for three to five days, EHEC strains showed increased tolerance to chlorine (adj. p < 0.05) compared to chlorine tolerance upon inoculation onto lettuce. This indicated that longer incubation on lettuce led to greater EHEC survival upon exposure to chlorine. Subsequent transcriptome analysis identified the upregulation of osmotic and oxidative stress response genes by EHEC after three and five days of incubation on pre-harvest lettuce. Assessing the physiological changes in EHEC that occur during association with pre-harvest lettuce is important for understanding how changing tolerance to post-harvest control measures may occur.
Highlights
In recent years, an increased number of gastroenteritis outbreaks have been linked to fresh fruits and vegetables
We enumerated viable cells of four strains of Escherichia coli (EHEC) (Table 1) on lettuce on days 0, 1, 3, and 5 post-inoculation to determine the effect of two seasonal (June and March) and relative humidity (RH) conditions
As the initial concentrations of each strain varied on the lettuce plants, the log decrease in CFU/g of lettuce was calculated for each day post-inoculation and used for comparison of survival across the different environmental conditions (Figure 1)
Summary
An increased number of gastroenteritis outbreaks have been linked to fresh fruits and vegetables. The Centers for Disease Control and Prevention estimated that produce accounted for almost half of all foodborne illnesses (46%) from 1998 to 2008, and of those illnesses linked to produce, 22% were attributed to leafy greens [6]. Usage of contaminated irrigation water, application of raw manure, direct fecal deposition by wildlife, and improper worker hygiene are a few ways in which the produce could become contaminated in the field [13]. Once these pathogens are deposited on produce, they are capable of surviving in this non-host environment for lengthy periods of time [14,15,16,17]
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