In Summary
- Listeria monocytogenes can be brought into processing environments from raw fish coming in for processing. Although the types of L. monocytogenes found on finished product is not normally the same as that found in raw fish entering the plants. This suggests that instead, finished product is contaminated frequently from L. monocytogenes persisting in the processing environment.
- Specific areas in the plant appear to be more susceptible to L. monocytogenes persistence. Including drains, skinning machines, brine injection units, and fish slicers.
- Over time, L. monocytogenes can adapt to the environment, making plant persistence more likely. Studies have found relatively few strains of L. monocytogenes, suggesting that L. mono resident strains represent the majority of end product contamination.
Plant resident, persistent L. monocytogenes
A review of the scientific literature relating to fish smoking revealed that some L. monocytogenes biotypes can persistently colonise plant processing environments for extended periods of several years. A number of studies have shown that plant resident strains are more likely to be isolated from the final products than biotypes present on batches of raw fish entering the plants. It is possible that raw fish could be the original source of the plant resident biotypes, although this cannot be irrefutably concluded from the literature available. Specific process stages and regions within smoked fish processing plants appear to be more susceptible to persistent colonisation by L. monocytogenes. Such reservoirs include drains, skinning machines, brine injection units and fish slicers.
A Polish study investigated L. monocytogenes contamination in a smoked fish processing plant (Mędrala et al, 2003). Seventy-one presumptive strains were isolated over a year from 152 samples of raw fish (salmon and sea trout) and the final product of vacuum-packed cold-smoked sliced salmon. Contamination of raw materials ranged between 4.3–15.4%, whereas final products were more significantly contaminated with a prevalence of up to 77.8%. Thus, the fish acquired L. monocytogenes during processing. The significantly higher prevalence in the finished product suggested that there may be persistent L. monocytogenes resident in the plant and acting as a contamination source for fish during processing. Although no environmental samples were taken from the plant by Mędrala and colleagues, PFGE was used to determine that the L. monocytogenes on the smoked final product were different from those entering the plant on raw fish. The authors believed there was evidence the product became heavily contaminated towards the end stages of processing (i. e. smoking, slicing, and/or packaging) by the plant-resident strains of L. monocytogenes. There was a dominant L. monocytogenes clone identified by PFGE (a form of bacterial typing). This dominant clone was further classified by additional restrictions into several closely-related strains. The authors discussed that the minor changes in the PFGE bands corresponded to a clone selection process and that DNA of the original strain has changed as the organism became more adapted to each of its colonised environmental niches. In keeping with the conclusions of other studies (Dauphin et al, 2001; Johansson et al, 1999), the Polish group believed that L. monocytogenes on raw material were a minor contributor to final product contamination at the plant they investigated. Furthermore, the authors speculated that L. monocytogenes became established in the plant environment as a consequence of ineffective cleaning and sanitation procedures (Mędrala et al, 2003).
There is other, more recent information that supports the opinions of Mędrala et al. (2003) regarding plant resident L. monocytogenes. Seven hundred seventy-eight samples of packaged smoked fish (774 smoked salmon and 4 smoked swordfish) on sale in Italy, from 50 different manufacturers, processing in 12 European Union countries, were purchased, and tested for L. monocytogenes (Acciari et al, 2017). 157 samples (20.1%) of samples tested positive for L. monocytogenes, with 26 samples (3.3%) exceeding the legal limit of 100 cfu/g for ready to eat foodstuffs (EC 2073/2005). The isolates were genotyped by pulsed field gel electrophoresis, and it was determined that eight main pulsotypes accounted for >70% of the isolates. Three of the main pulsotypes were exclusively from products processed by a single manufacturer. The study supplies further evidence to the already well-established fact that the main issue for L. monocytogenes contamination of product is plant resident strains, which was the reason that the majority of strains clustered by processor and the low amount of genetic diversity observed in the product isolates.
