Quantitative Microbial Risk Assessment for Escherichia coli O157 on Lettuce, Based on Survival Data from Controlled Studies in a Climate Chamber

Abstract

The aims of the study were to determine the survival of Escherichia coli O157 on lettuce as a function of temperature and light intensity, and to use that information in a screening-level quantitative microbial risk assessment (QMRA) in order to evaluate risk-reducing strategies including irrigation water quality guidelines, rinsing, and holding time between last irrigation and harvest. Iceberg lettuce was grown in a climate chamber and inoculated with E. coli O157. Bacterial numbers were determined with the standard plate count method after inoculation and 1, 2, 4, and 7 day(s) postinoculation. The experiments were carried out at 11, 18, and 25°C in light intensities of 0, 400, and 600 mmol (m(2))(-1) s(-1). There was a significant effect of temperature and light intensity on survival, with less bacteria isolated from lettuce incubated at 25 and 18°C compared with 11°C (P < 0.0001), and in light intensities of 400 and 600 mmol (m(2))(-1) s(-1) compared with 0 mmol (m(2))(-1) s(-1) (P < 0.001). The average log reductions after 1, 2, 4, and 7 day(s) were 1.14, 1.71, 2.04, and 3.0, respectively. The QMRA compared the relative risk with lettuce consumption from 20 scenarios. A stricter water quality guideline gave a mean fivefold risk reduction. Holding times of 1, 2, 4, and 7 day(s) reduced the risk 3, 8, 8, and 18 times, respectively, compared with harvest the same day as the last irrigation. Finally, rinsing lettuce for 15 s in cold tap water prior to consumption gave a sixfold risk reduction compared with eating unrinsed lettuce. Sensitivity analyses indicated that variation in bacterial inactivation had the most significant effect on the risk outcome. A QMRA determining the relative risks between scenarios reduces uncertainty and can provide risk managers with decision support.

Scientific Opinion on the risk posed by pathogens in food of non‐animal origin. Part 2 (Salmonella and Norovirus in leafy greens eaten raw as salads)

“…Because of the time between irrigation and harvest, pathogenic bacteria can be reduced in numbers by UV radiation from sunlight, drying, or competition with commensal microbiota (Brandl and Amundson, 2008;Ottoson et al, 2011). Increasing the interval between the time of contamination and the point of harvest significantly decreased the likelihood that pathogenic and non-pathogenic strains of E. coli would be present in the harvested product (Fonseca et al, 2011;Moyne et al, 2011).…”

“…There is no widespread agreement regarding the microbiological guidelines to be established for irrigation water but they should be preferably based upon risk assessment as recommended in WHO documents (http://www.who.int/water_sanitation_health/dwq/gdwq3rev/en/). An example of implementation is to be found in the Australian Water guidelines ( http://www.environment.gov.au/resource/national-waterquality-management-strategy-australian-guidelines-water-recycling-managing-0) and this may vary depending upon the time between irrigation and harvest and the type of irrigation method (Fonseca et al, 2011;Ottoson et al, 2011). In most cases, the enumeration of generic E. coli is used as an indicator organism as its presence relates to faecal pollution or failures in control measures.…”

Scientific Opinion on the risk posed by pathogens in food of non-animal origin. Part 1 (outbreak data analysis and risk ranking of food/pathogen combinations)

“…Foodborne pathogens present on the surface of fresh produce or of fresh cut produce may be inhibited by the indigenous microorganisms, and may be limited by available nutrients (Lopez-Velasco et al, 2012;Nguyen-The and Carlin, 2000); (Wood et al, 2010) demonstrated the bactericidal effect of UV radiation on enteric bacteria in the phyllosphere and there is also a significant effect of temperature and light intensity on survival of Escherichia coli O157 in lettuce (Ottoson et al, 2011). Prior to harvest, reduction in relative humidity caused death of Salmonella spp.…”

Belgian and Spanish Consumption Data and Consumer Handling Practices for Fresh Fruits and Vegetables Useful for Further Microbiological and Chemical Exposure Assessment

“…Complex contextual factors in fresh produce primary production, production processes, trade, and logistic organization of the fresh produce supply chain also can affect the food safety risk of fresh produce and their derived food products (8,32,38,46). Recent food contamination events have highlighted the importance of the evaluation of risks associated with the food chain through exposure assessment, a key step in risk assessment.…”