Evidence of an antilisterial factor induced by wounding of iceberg lettuce tissues

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Abstract
To examine the influence of wound-associated reactions in cut iceberg lettuce (Lactuca sativa L.) tissues on the fate of Listeria monocytogenes. Aqueous extracts prepared from shredded iceberg lettuce before and after storage in high oxygen permeability film were inoculated with L. monocytogenes. Listeria monocytogenes grew in extracts prepared from fresh lettuce. In contrast, inhibition ranging from arrested growth to a decline in cell viability was observed in extracts prepared from samples stored for 1-3 days. Similar behaviour was evident in lettuce shreds inoculated with 10(5) CFU g(-1)L. monocytogenes immediately after processing or after 3 days in storage. Heat treatment of the cut tissues at 47 degrees C for 3 min before storage diminished the inhibitory effect. The results provided evidence that an antilisterial factor or factors are released by wounded iceberg lettuce tissues. Antilisterial activity was mitigated by heat treatment of the lettuce. This study indicates that intrinsic factors associated with plant metabolism could play a significant role in the ecology of human pathogens in packaged horticultural products.