Fate of Listeria monocytogenes on Ready to Serve Lettuce ¹

The ability of Listeria monocytogenes to survive and grow on head lettuce obtained from a retail outlet over a period of 10 months was determined. Lettuce was torn into bite sized pieces, inoculated with L. monocytogenes ATCC 7644, placed into plastic bags, and held under a variety of storage conditions. Samples stored at 5°C and 12°C were subjected to aerobic plate count analysis, and levels of L. monocytogenes were determined immediately after inoculation and after 7 and 14 d of storage. Samples stored at 25°C were sampled after inoculation and after 4 and 8 h storage. Lettuce juice was inoculated, stored at 5°C and sampled as described for head lettuce. Aerobic plate counts on lettuce stored at 5°C and 12°C increased greatly during the 14 d of storage. Behavior of L. monocytogenes was variable. In most trials, numbers increased by several log cycles during 14 d of storage, but in several trials growth never occurred or did not persist for 14 d. The same general growth patterns were observed in lettuce held at 25°C. Aerobic plate counts increased by 1 or 2 log cycles and L. monocytogenes increased by 1 log cycle, except for occasional trials where the organisms did not grow or survive. Lettuce juice held at 5°C was also able to support growth of L. monocytogenes . L. monocytogenes serotype 1 was isolated from some uninoculated samples indicating that the organism was naturally present on some of the lettuce heads purchased from retail outlets.