Foodborne illnesses associated with the consumption of fresh produce such as raw vegetables have become a major health concern worldwide in recent years. In the present study, we investigated the possible routes of Salmonella contamination in leafy lettuce via soil during cultivation. After 10-week cultivation of lettuce plants in soils inoculated with S. Enteritidis expressing green fluorescent protein (SE-EGFP), the bacterium was detected in soil inoculated with >104 cfu/g and from most lettuce leaves cultivated in soils inoculated with >4.4 × 107 cfu/ g. As Salmonella was not detected in intact lettuce leaves or lettuce leaves with root injury cultivated in highly contaminated soils and after surface disinfection, the lettuce plants were not considered to internalize the bacterium. Overhead irrigation led to the contamination of one in 10 lettuce plants; however, all sets of three leaves of the plant were contaminated (>110 MPN/g). In an effort to prevent Salmonella contamination from soils, we investigated the effects of mulch on contamination levels during cultivation. Mulch effectively reduced Salmonella contamination levels of lettuce plants cultivated in highly contaminated soils.
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