Wageningen UR Library Catalogue

Eelco Franz

During the last decades an increasing number of foodborne disease outbreaks have been related to the consumption of vegetables. Contamination can occur by growing vegetables on fields enriched with animal manure harboring pathogens like E. coli O157:H7 and Salmonella Typhimurium. The objective of this thesis is to identify risk factors for the occurrence, survival and spread of these two pathogens in the primary production chain of lettuce, and to conduct a quantitative microbial exposure assessment for the contamination of lettuce.
The investigation of the potential impact of cattle diet on the survival of E. coli O157:H7 and two strains of Salmonella Typhimurium in manure revealed that pathogen decline rate was significantly increased when cows were fed a diet composed of straw compared to diet composed of highly digestible grass-maize silage. The decline rate was predominantly determined by the fiber content and pH of the manure (both positive relations). When manure was subsequently mixed with different soils on which lettuce seedlings were planted, the final pathogen loads did not result in the presence in or on the edible parts of lettuce.
In order to assess whether E. coli O157:H7 and Salmonella Typhimurium (MAE 110 and 119) can colonize the exterior and interior of lettuce plants and up to which densities, seedlings were grown in an inoculated hydroponic and soil system. With the hydroponic system only S. Typhimurium was able to colonize lettuce seedlings internally. With the soil system E. coli O157:H7 and both S. Typhimurium strains colonized the exterior and interior of lettuce seedlings up to high densities.
In a survey among organic and conventional dairy farms, the E. coli O157:H7 specific rfbE gene was detected at 52% of the farms, while Shiga-toxin genes were detected at 80% of the farms. The species diversity of Enterobacteriaceae was significantly lower in manure positive for rfbE, indicating the possible important role of the microbial diversity in STEC epidemiology. The survival time of an introduced E. coli O157:H7 was determined in all 25 manures and was negatively related to the numbers of generic coliforms present in the manure and the pH.
In order to identify risk factors for E. coli O157:H7 persistence in manure-amended soil the survival was determined in a set of 36 Dutch arable organic and conventional, sandy and loamy soils. Pathogen survival ranged from 54 to 105 days and the variability in survival times followed a logistic distribution. No differences in survival time were observed between both management and soil types. For sandy, loamy and conventional soils the variation in survival time was best explained by the level of dissolved organic carbon per unit biomass (positive relation). For organic soils the variation in survival time was best explained by the level of dissolved organic nitrogen (positive relation) and the microbial species diversity as determined by DGGE (negative relation). Survival increased with a field history of low-quality manure (artificial fertilizer and slurry) compared to high-quality manure application (farmyard manure and compost).
A quantitative microbial exposure assessment is presented to estimate the number of contaminated lettuce crops per hectare at the moment of harvest in the Netherlands. The baseline model estimated an average presence of 0.34 (5th percentile: 0.03, 95th percentile 0.92) contaminated lettuce plants per hectare (on average 1 in 3.27•105 produced heads in the Netherlands). Sensitivity analysis revealed that the likelihood of exposure was most sensitive to the prevalence of contaminated manure, the manure storage time and the initial density of E. coli O157:H7 in naturally contaminated manure. A traditional organic production scenario resulted in a 71% reduction in the expected number of contaminated lettuce plants while an intensive conventional scenario resulted in an increase of 62%.

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