Sources of Listeria monocytogenes contamination in a cold-smoked rainbow trout processing plant detected by pulsed-field gel electrophoresis typing

Sites of Listeria monocytogenes contamination in a cold-smoked rainbow trou t (Oncorhynchus mykiss) processing plant were detected by sampling the prod uction line, environment, and fish at different production stages. Two lots were monitored. The frequency of raw fish samples containing L. monocytoge nes was low. During processing, the frequency of fish contaminated with L. monocytogenes clearly rose after brining, and the most contaminated sites o f the processing plant were the brining and postbrining areas. A total of 3 03 isolates from the raw fish, product, and the environment were characteri zed by pulsed-field gel electrophoresis (PFGE), PFGE yielded nine pulsotype s, which formed four clusters. The predominating L, monocytogenes pulsotype s of the final product were associated with brining and slicing, whereas co ntaminants of raw fish were not detected in the final product. Air-mediated contamination in the plant could not be proved. In accordance with these r esults, an L, monocytogenes eradication program was planned. The use of hot steam, hot air, and hot water seemed to be useful in eliminating L,, monoc ytogenes, None of the control samples taken in the 5 months after the eradi cation program was implemented contained L, monocytogenes.