Assessment of Escherichia coli O157:H7 transference from soil to iceberg lettuce via a contaminated field coring harvesting knife

The potential for lettuce field-coring harvesting knives to cross-contaminate lettuce heads with pathogens was evaluated. Rings and blades of the harvest knives artificially contaminated with Escherichia coli O157:H7 (EHEC), were used to core three successive heads of iceberg lettuce. The coring rings and blades were inoculated by dipping into soils containing EHEC at concentration ranges of 1-10⁵ MPN/g soil. Factors that influenced EHEC transference from soil to iceberg lettuce via contaminated coring knife blade, included water content (WC) of clay and sandy soils, EHEC concentration, and degree of blade contact (stem, medium, and heavy) with edible tissue. High moisture content clay soil was positively associated with high pathogen transference. No EHEC were detected on any cut heads when clay soil contaminated with 10⁵ MPN/g EHEC had WC of 20% or less, or when the knife blade was dipped into sandy soil contaminated with EHEC at the same level, regardless of percent WC. The extent to which the harvesting knife blade cut across edible lettuce tissues was also an important factor in the amount of pathogen transference that occurred. EHEC were detectable on first and second sequentially cut lettuce heads when medium-contact was made between knife blade and edible tissues and on all three sequentially cut lettuce heads using the heavy-contact cutting scenario, when the blade was contaminated with 10⁴ cfu/g EHEC in clay soil (25% WC). However, when the blade, contaminated at the same soil EHEC level, was used to cut only the stem and had no contact with the edible portion of the lettuce head, no pathogen transference was detected. Under the current CIF harvesting practice, the cutting blade has a higher potential than the coring ring to be contaminated by the soil, but less opportunity to transfer pathogens to harvested lettuce. However, once contaminated, the coring ring has much higher potential than the blade to transfer pathogens to the harvested lettuce.