Tracing Listeria monocytogenes isolates from cold-smoked salmon and its processing environment in Iceland using pulsed-field gel electrophoresis

Listeria spp. and Listeria monocytogenes contamination of cold-smoked salmon (n=125) and its processing environment (n=522) were evaluated during surveys conducted in 1997-1998 and 2001 as well as in samples of final products analysed in 2001. The overall frequencies of Listeria spp. and L. monocytogenes in samples from all sources were 15.1% and 11.3%, respectively, but the incidence of L. monocytogenes in cold-smoked salmon final products was only 4%. A total of 201 L. monocytogenes isolates were characterised by Pulsed-Field Gel Electrophoresis (PFGE) in order to trace L. monocytogenes contamination in the processing plants. The combination of AscI and ApaI macrorestriction patterns yielded 24 different pulsotypes in 6 plants. One pulsotype observed by AscI restriction digestion comprised 148 of the 167 typed isolates from two processing plants. Two other pulsotypes predominated in samples from raw material, processing environments and final products. The results indicate that raw material, floors, and drains are potential sources of the L. monocytogenes found on cold-smoked salmon products. This highlights the need to readdress the design and cleaning of processing plants and equipment, and staff behavior. Hindering the introduction into and spread of the organism through the processing environment is necessary to avoid jeopardizing safety of the final product.