Survival of Listeria monocytogenes on Lettuce With and Without Injury

Listeria monocytogenes (Lm) is a bacterium that causes listeriosis and outbreaks have been linked to the consumption of fresh produce. The purpose of this project was to establish if there is potential for Lm to persist on lettuce leaves. It also evaluated whether there is a difference in survival of this pathogen on injured and uninjured lettuce leaves. This study used Lm strain LS1061; a spontaneous mutation resulting in a rifampicin resistant strain. LS1016 is a serotype 4b isolate from the caramel-covered apple outbreak. Three conditions were evaluated: tear injury and 10μL Lm culture, no injury and 10μL Lm culture on the back of the leaf, no injured plus Lm on the midvein of the leaf. Three independent trials for Romaine lettuce and two independent trials for Iceberg lettuce were conducted. When plants were about half grown, lettuce leaves were injured by tearing a portion of the leaf, about an inch (2.5cm) from the tip to expose the inner tissues. Romaine trials showed that by day 4, positivity on all samples did not have major decline. Decline to ~50% or under was seen at Day 14. Injured Romaine had a high percentage (≥40%) of positive samples through Day 21. Like Romaine, Iceberg trials did not fall to 50% or lower positives until Day 14. Injured Iceberg and “cup” had positive samples through Day 21.