RESUMEN
Contamination routes of Listeria monocytogenes were examined for 3 years in an Iberian pork-processing plant that produced high-quality ready-to-eat meat products. Molecular subtypes of L. monocytogenes were determined by polymerase chain reaction-based serotyping and pulsed-field gel electrophoresis (PFGE) restriction analysis. A total of 541 L. monocytogenes isolates were recovered from the environment and equipment (n = 165), carcasses (n = 28), raw products (n = 321), and dry-cured products (n = 27). Only 29 different PFGE types were identified, 3 of which were repeatedly found to be persistent types and accounted for 73% of the isolates. One PFGE type dominated (45% of the isolates) and was mostly recovered from intermediate manufactured products and the environment of the manufacturing area. L. monocytogenes persistence appeared strongly linked to the manufacture of products and not to its sustained entrance with the raw material. Some clones were found to survive in the manufacturing area for 3 years. Controlling the contamination of raw ingredients, improving the compartmentalization, and changing the cleaning protocols resulted in reduced prevalence rates of L. monocytogenes on products; two persistent PFGE types were eliminated from the processing plant, although eradication of other adapted strains has not been achieved.