Abstract
Field trials were conducted in July–August and October 2012 to quantify the inactivation rate of Escherichia coli O157:H7 when mixed with fecal slurry and applied to romaine lettuce leaves. Lettuce was grown under commercial conditions in Salinas Valley, California. One-half milliliter of rabbit, chicken, or pig fecal slurry, containing an average of 4.05 × 107 CFU E. coli O157:H7 (C0), was inoculated onto the upper (adaxial) surface of a lower leaf on 288 heads of lettuce per trial immediately following a 2.5 h irrigation event. To estimate the bacterial inactivation rate as a function of time, fecal matrix, irrigation and seasonal climate effects, sets of lettuce heads (n = 28) were sampled each day over 10 days and the concentration of E. coli O157:H7 (Ct) determined. E. coli O157:H7 was detected on 100% of heads during the 10-day duration, with concentrations ranging from ≤340 MPN/head (∼5-log reduction) to >3.45 × 1012 MPN/head (∼5-log growth). Relative to C0, on day 10 (Ct = 12) we observed an overall 2.6-log and 3.2-log mean reduction of E. coli O157:H7 in July and October, respectively. However, we observed relative maximum concentrations due to bacterial growth on day 6 (maximum Ct = 8) apparently stimulated by foliar irrigation on day 5. From this maximum there was a mean 5.3-log and 5.1-log reduction by day 10 (Ct = 12) for the July and October trials, respectively. This study provides insight into the inactivation and growth kinetics of E. coli O157:H7 on romaine lettuce leaves under natural field conditions. This study provides evidence that harvesting within 24 h post irrigation has the potential to increase the concentration of E. coli O157:H7 contamination, if present on heads of romaine lettuce; foliar irrigation can temporarily stimulate substantial regrowth of E. coli O157:H7.
Highlights
Consumption of contaminated fresh produce continues to be a significant cause of foodborne illness in the US (Richardson et al, 2017)
Despite a considerable amount of effort by private industry, academia, and governmental agencies to fully understand the core mechanisms and contributing factors responsible for microbial contamination in the preharvest environment, it is not known what proportion of the annual foodborne incidence of Escherichia coli O157:H7 or other pathogenic enteric bacteria are associated with produce is due to contaminated irrigation water, wildlife intrusion, or the use manure-based soil amendments in the preharvest production environment
Our findings regarding regrowth of E. coli O157: H7 following exposure to either higher levels of leaf wetness or foliar irrigation are consistent with the findings reported by Scherber, Schottel & Aksan (2009) who observed bacterial cell membrane reconstruction leading to cell proliferation and colony formation within 1-day of adequate rehydration
Summary
Consumption of contaminated fresh produce continues to be a significant cause of foodborne illness in the US (Richardson et al, 2017). While contaminated irrigation water can act as a vehicle to directly transfer pathogens onto crops (Barker-Reid et al, 2009; Erickson et al, 2010; Fonseca et al, 2011; Solomon, Pang & Matthews, 2003; Solomon, Potenski & Matthews, 2002a; Wachtel, Whitehand & Mandrell, 2002), it has been shown to transfer pathogens from either manure or manure-amended soil (Franz, Semenov & Van Bruggen, 2008; Mootian, Wu & Matthews, 2009; Solomon, Yaron & Matthews, 2002b); and simulated wildlife scat onto nearby lettuce heads (Atwill et al, 2015; Weller et al, 2017a)
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