Application of molecular technique for risk assessment of Listeria monocytogenes in a frozen cooked chicken processing plant

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Chirapiphat Phraephaisarn
Suwimon Keeratipibul


L. monocytogenes, the causative agent of listeriosis in humans and animals, is one of the most concerned pathogen affecting frozen ready-to-eat food industry worldwide. It could remain in the processing environment for years and cause cross-contamination into the products. The objectives of this study were to evaluate occurrence of Listeria spp., and to investigate contamination sources of L. monocytogenes. In this study, a total of 100 environmental swabs, and 100 finished product samples were analyzed by a conventional method and subjected to multilocus variable number of tandem repeat analysis (MLVA) for subtyping L. monocytogenes. From a total of 100 environmental swab samples taken from before (50 samples) and after (50 samples) cleaning, 28 (56%) and 16 (32%) samples were positive for Listeria spp., respectively. The occurrence of L. monocytogenes was 20%. Meanwhile, 20 samples the finished products were positive for Listeria spp., accounting for 20% occurrence. The occurrence of L. monocytogenes was 6%. Then, the MLVA with selected 9 VNTR loci was applied to evaluate sources of contamination of 25 L. monocytogenes isolates found in the environmental swabs and product samples. L. monocytogenes isolates were classified into 3 strains which were LM1, LM2, and LM3. In the environmental swabs, the LM1 was the dominant strain represented by 74% of all L. monocytogenes positive samples. Other L. monocytogenes strains were LM2 (5%) and LM3 (21%). However, only LM1 strain was found in both finished product and environmental swab samples. There was correlation between L. monocytogenes strain LM1 found in processing environment and in finished products. The cross contamination from processing environment to finished products may occur due to improper cleaning procedure and staff straining. In this study, it clearly showed that the MLVA can be used as a useful tool for investigation of L. monocytogenes contamination in food products.


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