In Summary
- Listeria monocytogenes can be brought into processing environments from raw fish coming in for processing. L. monocytogenes can then persist within the processing environment and become plant resident strains. It is more common for plant resident strains to be the primary source of contamination for finished product, rather than contaminated raw fish entering the plant.
- Studies have found that L. monocytogenes persistence in plant drains is a risk factor for contamination in finished product.
- Use of pressured water to clean surfaces is a risk factor for spreading of L. monocytogenes.
- Poor cleaning and sanitation has been shown to lead to significant levels of L. monocytogenes contamination, across many food contact surfaces. With certain areas, such as screw threads, underneath rivet heads, and cracks in concrete being difficult to clean with chemical sanitisers, with heat disinfection being preferable for these niches.
There is evidence that although L. monocytogenes can be brought into a processing plant on fish, the L. monocytogenes strains isolated from final product are different to those present on the raw materials (Autio et al., 1999). A commonly reported scenario is that L. monocytogenes enter a processing plant, become persistently established in the plant environment and from those environmental niches contaminate a large proportion of the food manufactured in the plant (Tompkin, 1999). The presence of L. monocytogenes in the drains was found to be a sensitive predictor for L. monocytogenes isolations from finished smoked salmon (Rørvik et al, 1997). Adding further supportive evidence to the findings of Rørvik et al. (1997) a role for drains in the contamination of smoked fish with L. monocytogeneswas also reported by Hoffman et al. (2003). Environmental and final product samples were taken in two commercial processing plants over a two-month period and the isolates were ribotyped (Hoffman et al., 2003). The Hoffman study showed there were indistinguishable link to glossary entry ribotypes from the plant drains and isolates from the final produce and concluded that the drains in smoked fish processing plants can be the source of L. monocytogenes contamination of final product. Similar conclusions were made by Autio et al. (1999) and Hoffman et al. (2003).
Berrang and Franks (2012) inoculated L. innocua into floor drains as a model for L. monocytogenes. The drains were then sprayed with a burst of low pressure (~70kPa) tap water for two seconds and any airborne Listeria generated by the water impact were captured using an impaction sampler and also settle plates placed in the area around the drains. The work had robust replication and the authors observed that Listeria spp. was recovered from settle plates on the floor at distances of up to 4 metres from the sprayed drain and from walls at heights as high as 2.4 metres above the floor.
Berrang et al. (2013) also undertook follow on work relating to drains. As before, model floor drains were inoculated with L. innocua. The inoculated drains were sprayed with a low-pressure water hose. Chicken breast fillets (uncooked) were left uncovered on a table 2.4 m away from the drain. After 10 minutes, a different set of fillets were indirectly exposed by placement on the same table. The results showed 18 L. innocua cells were transferred on to each of the directly exposed fillets. The indirectly exposed fillets were also contaminated with L. innocua at a level of around nine cells per fillet.
In combination, the Berrang studies demonstrate the hazards of using hoses in a processing hall for mid-shift cleaning when there is product present. Drain splash can be a route for the contamination of ceilings and any issues with condensation can cause contaminated water to drip onto food and food contact surfaces. An explanation of why condensation forms can be found in the condensation resource page.
Routine surveillance of smoked fish in Finland showed a higher-than-expected prevalence of L. monocytogenes (Johansson et al, 1999). Subsequent investigation revealed that the majority of the contaminated fish had been smoked at a single plant. The researchers investigated further by taking 200 samples from the plant environment and final products of the processor over two extended periods of time. In addition, six fish farms providing raw material fish to the plant were also sampled. The L. monocytogenes isolates were all typed by a DNA fingerprinting method. The processing environment was found to be contaminated with at least four of the strains isolated from the smoked fish. The areas that were contaminated with the same L. monocytogenes that was isolated from the final product were the plant drains, the skinning and salting equipment, conveyor belts and the packing machinery. The farm samples and raw material fish all tested negative for L. monocytogenes. The authors hypothesised that difficulties cleaning and sanitising effectively without disassembling the salting and slicing equipment was the main reason for the equipment contamination.
The main finding of the Johansson et al. (1999) study was that plant resident L. monocytogenes strains were the primary source of contamination for finished product, rather than contaminated raw fish entering the plant. That finding has subsequently been reported by a number of authors (Autio et al, 1999; Vogel 2001a, Dauphin et al, 2001; Carpentier and Cerf, 2011; Skowron et al, 2018; Skowron et al. 2019). A well-established route of contamination is that L. monocytogenes enters plants on raw fish and becomes established in the plant environment as persistent strains. Imperfect cleaning and sanitation are frequently implicated as a factor that helps L. monocytogenes become established (Johansson et al. 1999, Autio et al. 1999, Vogel 2001a, Dauphin et al. 2001, Vongkamjan et al. 2013). It is far more common for plant resident persistent strains to contaminate finished product than L. monocytogenes present on raw fish (Acciari et al. 2017). There is evidence that indicates residency in fish plants can persist for more than 70 years (Harrand et al. 2020).
The specific detection of L. monocytogenes can be difficult if samples are taken soon after cleaning and disinfection. Bacterial cells can sometimes only be injured and not killed by the chemicals used for cleaning and sanitation. Injured L. monocytogenes cells can still be alive, but not multiply and are therefore not easily detectable (NSW, 2008). In addition, L. monocytogenes are adept at becoming established in environmental niches such as screw threads, underneath rivet heads and cracks in concrete. It is near impossible to clean such areas properly with chemicals and one reason why heat is the preferred method for many processors for the decontamination of equipment known to be colonised by L. monocytogenes. 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; Berrang et al. 2012). 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.
