Distribution of Listeria monocytogenes molecular subtypes among human and food isolates from New York State shows persistence of human disease--associated Listeria monocytogenes strains in retail environments.

While there is considerable information available regarding Listeria monocytogenes contamination patterns in food processing plants, our understanding of L. monocytogenes contamination and transmission in retail operations is limited. We characterized 125 food, 40 environmental, and 342 human clinical L. monocytogenes isolates collected in New York State from 1997 to 2002 using automated ribotyping and hly allelic variation. All environmental isolates were obtained from retail establishments and the majority of food isolates (98 isolates) were obtained from foods that were prepared or handled at retail. Overall, food and/or environmental isolates from 50 different retail establishments were characterized. The 125 food and 40 environmental isolates were differentiated into 29 and 10 ribotypes, respectively. For 16 retail establishments, we found evidence for persistence of one or more specific L. monocytogenes strains as indicated by isolation of the same EcoRI ribotype from food or environmental samples collected in a given establishment on different days. The human isolates were differentiated into 48 ribotypes. Statistical analyses showed that two ribotypes were significantly (P < 0.0001) more common among food isolates as compared with human isolates. However, a total of 17 ribotypes found among the human clinical isolates were also found among the food and environmental isolates. We conclude that L. monocytogenes, including subtypes that have been linked to human disease, can persist in retail environments. Implementation of Listeria control procedures in retail operations, which process and handle products that permit the growth of L. monocytogenes, are thus a critical component of a farm-to-table L. monocytogenes control program.