We have applied molecular approaches, including PCR-based detection strategies and DNA fingerprinting methods, to study the ecology of Listeria monocytogenes in food processing environments. A total of 531 samples, including raw fish, fish during the cold-smoking process, finished product, and environmental samples, were collected from three smoked fish processing facilities during five visits to each facility. A total of 95 (17.9%) of the samples tested positive for L. monocytogenes using a commercial PCR system (BAX for Screening/Listeria monocytogenes), including 57 (27.7%) environmental samples (n ‫؍‬ 206), 8 (7.8%) raw material samples (n ‫؍‬ 102), 23 (18.1%) samples from fish in various stages of processing(n ‫؍‬ 127), and 7 (7.3%) finished product samples (n ‫؍‬ 96). L. monocytogenes was isolated from 85 samples (16.0%) using culture methods. Used in conjunction with a 48-h enrichment in Listeria Enrichment Broth, the PCR system had a sensitivity of 91.8% and a specificity of 96.2%. To track the origin and spread of L. monocytogenes, isolates were fingerprinted by automated ribotyping. Fifteen different ribotypes were identified among 85 isolates tested. Ribotyping data established possible contamination patterns, implicating raw materials and the processing environment as potential sources of finished product contamination. Analysis of the distribution of ribotypes revealed that each processing facility had a unique contamination pattern and that specific ribotypes persisted in the environments of two facilities over time (P < 0.0006). We conclude that application of molecular approaches can provide critical information on the ecology of different L. monocytogenes strains in food processing environments. This information can be used to develop practical recommendations for improved control of this important foodborne pathogen in the food industry.The advent of molecular methodology has revolutionized our ability to investigate and understand microbial ecology, offering new and unique opportunities to explore the ecology of food-borne pathogens, including Listeria monocytogenes, throughout the food chain and in the food processing environment. Highly discriminatory molecular typing methods, including multilocus enzyme electrophoresis, pulsed-field gel electrophoresis (PFGE), random amplification of polymorphic DNA, ribotyping, and phage typing, have been successfully applied to investigations of contamination patterns in foods and in the food processing environment and are increasingly used for surveillance of human disease cases and for tracking of outbreak sources (2-4, 7, 12, 26, 34, 36, 37, 39). While each method provides discriminatory differentiation of L. monocytogenes subtypes, highly automated and standardized methods provide a simplified approach to molecular subtyping and data analysis. The RiboPrinter Microbial Characterization System (Qualicon, Inc., Wilmington, Del.) is one example of such an approach. This system is based on ribotyping, a subtyping method based upon scoring restriction poly...

The cold-smoked fish industry was used as a model for the development of a system for monitoring Listeria spp. in foods and in the food processing environment. A total of 214 samples including raw fish, fish during the cold-smoking process, finished product, and environmental samples were collected from three processing facilities over two visits to each facility. Samples were screened for Listeria spp. using the BAX for Screening/genus Listeria polymerase chain reaction system (PCR) and by culture. Listeria spp., confirmed by the API Listeria test strip or by a PCR assay targeting the L. monocytogenes hlyA gene, were isolated from a total of 89 (41.6%) samples. Of these, 80 samples also tested positive for Listeria spp. using the BAX system. Specifically, 42 (55.3%) environmental samples (n = 76), 11 (25.6%) raw materials samples (n = 43), 20 (35.1%) samples from fish in various stages of processing (n = 57), and 7 (18.4%) finished product samples (n = 38) tested positive for Listeria spp. using the BAX system. Five (4.0%) of the 125 culture-negative samples yielded BAX system-positive results. Listeria isolates from each of nine culture-positive/BAX system-negative samples yielded a positive reaction when tested in pure culture by the BAX system, suggesting that our false-negative results were likely due to the presence of low Listeria numbers in the initial enrichment as opposed to nonreacting isolates. The employment of alternative enrichment protocols, such as the two-step enrichment recommended by the manufacturer, may increase the sensitivity of the assay.