Contamination of cooked peeled shrimp (Pandalus borealis) by Listeria monocytogenes during processing at two processing plants.

Listeria spp. and Listeria monocytogenes contamination was evaluated in cooked peeled shrimp (final or semifinal product, 82 samples) and the shrimp-processing environment (two plants, 613 samples) in eight surveys conducted from 1998 through 2001. Listeria was detected in 12.5% (78) of the 695 samples (11.2% of the samples were positive for L. monocytogenes), but none of the samples of final product contained Listeria. One hundred seventy-two L. monocytogenes isolates were characterized by pulsed-field gel electrophoresis. Cleavage with macrorestriction enzymes AscI and ApaI yielded 14 different pulsotypes in the plants; two types were dominant, one in each plant. Sixty-three of the 106 isolates in plant A and 43 of the 66 isolates in plant B were of the dominant types. Certain strains, mainly of serotypes 1/2c and 4b and pulsotypes 1A and 2H, were persistent for long periods in both plants. Adaptation of good hygienic practices in the processing plants, including strict rules concerning traffic of staff and equipment, and existing hygienic requirements appeared to be effective in preventing contamination between areas within plants and in the final product. The persistence of Listeria strains in these two processing plants indicates the importance of detecting the places in the processing environment (e.g., transporters, equipment, floors, and drains) where L. monocytogenes can survive so that cleaning and disinfection efforts can be directed to such niches.

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.

Characterization of Listeria monocytogenes isolates from the meat, poultry and seafood industries by automated ribotyping.

A total of 564 Listeria monocytogenes isolates were characterized by automated ribotyping. The samples were taken from equipment, personnel and the environment after cleaning procedures and during food processing, as well as from raw materials and products from six meat, two poultry and five seafood processing plants located in the Faroe Islands, Finland, Iceland, Norway and Sweden. Altogether, 25 different ribotypes (RTs) were generated. Two RTs occurred in the samples from all three food sectors--meat, poultry and seafood. Four RTs occurred in meat and poultry plant samples and other four RTs occurred in meat and seafood plant samples. Five RTs occurred only in meat plant samples, five only in poultry plant samples and five only in seafood plant samples. Eight of the thirteen plants had their own in-house L. monocytogenes ribotype. There was geographical differences between the RTs, but no correlation between RTs and food sectors was detected. The discrimination power of automated ribotyping was satisfactory to trace the contamination sources in the food processing plants clearly indicating the sites at which improved cleaning procedures were necessary. In addition, it was possible to screen a large number of isolates with two instruments located at different institutes and to make a reliable combination of the results.