Occurrence and typing of Listeria monocytogenes strains in retail vacuum-packed fish products and in a production plant

One hundred and ten samples of ready-to-eat, vacuum-packed, smoked and cold -salted fish products were collected from retail outlets in southern Finlan d during 1996 for examination of the occurrence and level of Listeria monoc ytogenes. The samples originated from 12 producers. Positive samples with l evels exceeding 100 CFU/g were encountered mainly in one of the producers ( no. 8). Therefore, 200 samples from the plant and the products of this prod ucer were studied during August-September 1996 and May-September 1997, as w ell as 55 samples from the six fish farms providing raw material fish to th is plant, during September 1997-January 1998. The isolates were characteris ed by serotyping and pulsed-field gel electrophoresis (PFGE). L. monocytoge nes was isolated in 20% (22/110) of the samples from the retail market, ori ginating from 6 producers. Ten of these positive samples contained L. monoc ytogenes at > 100 CFU/g (maximum 1.37 x 10(4) CFU/g). Seventeen percent (5/ 30) of cold-smoked and 50% (16/32) of cold-salted rainbow trout samples wer e contaminated. Only one hot-smoked fish product (2%) was found to be posit ive by enrichment. Nineteen (86%) of the strains isolated from the retail s amples belonged to serovar 1/2a and three (14%) to serovar 4b. In further s tudies the production line of plant no. 8 was found to be contaminated. All of isolates from up until autumn, 1997 both the products and the productio n plant were serovar 1/2a; thereafter one strain of 4b and one of 1/2 (H-an tigen untypeable) were isolated from the plant. The samples from raw materi al fish were all negative for L. monocytogenes. The samples from retail mar ket fell into seven PFGE types. Five and nine PFGE types, respectively, wer e found from the products and the plant of producer no. 8, PFGE type A was detected from the retail products of four producers and was also dominant a mong the isolates from production plant no. 8. PFGE type A was the only one found repeatedly from skinning, salting and slicing units as well as from products throughout the whole period. PFGE proved to be a powerful tool for studying contamination points and routes in the production plant. The meas ures based on hazard analysis critical control points (HACCP) program resul ted in L. monocytogenes negative samples at production plant no. 8 from the beginning of January 1998. (C) 1999 Elsevier Science B.V. All rights reser ved.