In Summary
- Testing of processing environments for foods (such as fish products) that support growth of L. monocytogenes is a legal requirement.
- Fish can be a source of L. monocytogenes, especially in the gills, where L. monocytogenes can be concentrated.
- Collection of environmental samples before commencement of processing is considered a good practice. This is because it checks cleaning effectiveness and provides assurance that equipment was in an appropriate condition for processing.
- Sample collection during processing is also a good practice.
- With all isolates of L. monocytogenes that are obtained from a processing environment, identifying the strain (i.e. via typing) is important, to identify possible historical persistence of the same L. monocytogenes strain.
When fish are contaminated with low numbers of L. monocytogenes or when contamination is sporadic, the literature suggests that there may be merit in selectively sampling the gills. The reason being that there is evidence that gill-filtered water containing small quantities of L. monocytogenes will concentrate the bacterium on the gill gas-exchange surfaces (Miettinen and Wirtanen, 2005; Jami et al. 2014). Thus, if L. monocytogenes is present, it will be most easily isolated from the gills.
Lie and colleagues (1989) concluded that lysozymes play an important role in infection control in fish. The majority of the external surfaces of fish are coated in mucus containing lysozyme. Lysozyme damages the structure of Listeria cells, injuring, or even killing, the bacteria. The relatively lower concentrations of lysozyme in gills, coupled with the potential concentration of pathogens during filtration of large quantities of water, may be a contributory factor to reports of higher isolations from these organs.
Collection of samples after cleaning and before the commencement of processing is considered a good practice because it checks cleaning effectiveness and provides FBOs with an assurance their premises and equipment were in an appropriate condition for processing. Good practice is a status that is distinct from a legal obligation. Collection of samples during processing is also a good practice because in combination with typing, it can let a FBO know if they have a plant resident strain. There is no legal obligation for FBOs to type strains isolated during processing. The typing is important because it differentiates between raw fish strains and plant resident strains (i. e. resident strain profiles that crop up repeatedly).
The collection of samples pre-processing, and post-processing tell you different things, but the things they tell you are both important and so are good practices.
It is possible cells can be dislodged from environmental niches during processing when equipment is being used or water is inadvertently splashed from cracked drains (Tompkin 2002). In order to establish whether there is an issue with persistent L. monocytogenes, any isolates need to be typed. Commonly typing generates a DNA fingerprint and the same fingerprint occurring over a long period of time indicates a potential persistent colonisation.
It is also a legal requirement (Article 5, regulation 2073/2005) that testing of processing environment(s) be undertaken for foods such as fish that support the growth of L. monocytogenes.
