Isolation, detection, and partial characterization of Listeria in smoked seafood

Dillon, Ronda Marie (1992) Isolation, detection, and partial characterization of Listeria in smoked seafood. Masters thesis, Memorial University of Newfoundland.

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Abstract

Over a one-year period, 258 samples of smoked fish products were obtained from retail outlets in Newfoundland and processed for Listeria using the listeria isolation protocol adopted by Health and Welfare Canada. Direct plating was also carried out. Listeria spp. were isolated from 43 of 258 (16.7%) samples, with hot smoked products yielding 25.4% (36/142) of the isolates, and cold smoked products yielding 6% (7/116). Cod had the highest rate of Listeria contamination at 46.7%. Of the 43 Listeria spp. isolated, 18 (41.9%) were L. innocua, 13 (30.2%) were L. welshimeri, and 12 (27.7%) were L. monocytogenes. Eleven isolates of L. monocytogenes were serotype 1/2, and one was 4b. There was an indication that Listeria spp., particularly L. monocytogenes, occurred more commonly during the cooler months. All Listeria isolates were detected after 48h primary enrichment. The secondary enrichment step failed to enhance the isolation rate. Direct plating yielded only 26% of the total isolates. PALCAM, a listeria selective agar, proved more efficient in recovering Listeria spp. from smoked fish products. -- Multilocus enzyme electrophoresis is an epidemiological tool applied in this study to Listeria strains isolated from smoked seafoods to determine if the strains were from a common contamination source. The 57 strains analyzed resulted in 57 distinct electrophoretic types (ETs) indicating many sources of contamination. Major divisions among the four different Listeria spp. (L. innocua, L. monocytogenes, L. welshimeri, L. ivanovii) occurred at genetic distances of 0.72 to 0.85. All 14 enzymes were polymorphic and had a mean genetic diversity of 0.683. The L. monocytogenes strains were not of a single clone nor were they of the same ETs as the two human isolates. This indicates the great need for better sanitation practices in the food processing environment. -- Experiments were conducted to study the effects of the cold smoking process and storage on the organism in artificially inoculated cod fillets. In three trials, cod fillets were surface inoculated with L. monocytogenes, cold smoked, vacuum-packaged, stored at 4°C for 3 weeks or at -20°C for 3 months, and quantitatively processed for Listeria using the three-tube most probable number method (MPN). The initial inoculation concentrations were 10⁵ MPN/g in trial 1, 10² MPN/g in trial 2, and 10⁰ MPN/g in trial 3. The organism, relatively stable during the smoking process, increased with storage at 4°C, but decreased in cell number at -20°C. Growth was very slow in trial 3 possibly because of the low inoculum level. Hence, L. monocytogenes contamination in smoked seafoods, not cooked prior to consumption, may pose a health risk to the consumer.

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