Use of PFGE typing for tracing contamination with Listeria monocytogenes in three cold-smoked salmon processing plants

The sites of Listeria monocytogenes contamination in three cold-smoked salm on (Salmo salar) processing plants were detected by sampling salmon and the plant's environment and equipment at different production stages. Of the 1 41 samples collected from three processing plants, 59 (42%) were contaminat ed with L. monocytogenes. The rates of contamination varied as to the plant and the sample source. L. monocytogenes isolates from 17 various contamina ted seafood products (fresh, frozen and smoked fishes, cooked mussels) were also studied. A total of 155 isolates from the three plants and the variou s seafoods were characterized by genomic macrorestriction using ApaI and Sm aI with pulsed-field gel electrophoresis (PFGE) and 82 isolates were seroty ped. Macrorestriction yielded 20 pulsotypes and serotyping yielded four ser ovars: 1/2a, 1/2b, 1/2c, 4b (or e), with 77 (93%) belonging to serovar 1/2a . One clone of L, monocytogenes predominated and persisted in plant I and w as the only pulsotype detected in the final product although it was not iso lated from raw salmon. No L, monocytogenes was detected in the smoked skinn ed salmon processed in plant II, even though 87% of the raw salmon was cont aminated. All the smoked salmon samples collected in plant III were contami nated with a unique clone of L. monocytogenes, which may have occurred duri ng slicing. In the three plants, the contamination of final products did no t seem to originate from the L. monocytogenes present on raw salmon, but fr om the processing environment. (C) 2001 Elsevier Science B.V. All rights re served.