2018 Volume 24 Issue 5 Pages 911-920
Outbreaks of foodborne illness caused by Listeria monocytogenes in or on fresh produce have been reported. Several contamination routes have been proposed and the cultivation process is one of the suspected routes. Leaf lettuce is one of the most important fresh produce products. In the present paper, we investigated the possible routes of L. monocytogenes contamination in leaf lettuce during cultivation. Leaf lettuce was cultivated in soils inoculated with cocktails of three isolates of L. monocytogenes belonging to different serotypes (1/2a, 1/2b, and 4b). The viability and injury state of L. monocytogenes in soils, and bacteria survival in or on leaf lettuce were investigated during 10 weeks of cultivation. Soils were artificially contaminated with L. monocytogenes at levels of 4, 6 or 8 log CFU/g, followed by cultivation of leaf lettuce in the contaminated soils. Populations of L. monocytogenes in the soil decreased to less than the detection limit (< 2 log CFU/g) at 8–10 weeks after the start of cultivation. L. monocytogenes was detected in some harvested leaf lettuce leaves at low levels, almost equivalent to the detection limit. As L. monocytogenes was not detected in the leaves of leaf lettuce plants cultivated in highly-contaminated soils after surface disinfection, the possibility of internalization of L. monocytogenes into leaf lettuce plants was considered low. Spraying of water contaminated with bacteria at greater than 3.2 log CFU/plant led to the survival of bacteria on the leaf lettuce leaves even after seven days. Furthermore, leaf damage prolonged the survival period of bacteria on the leaves.
