Occurrence and control of Listeria monocytogenes in the beef and pork supply chain

Despite the efforts already made by the sector, slaughterhouses and cutting plants are still confronted with the foodborne pathogen Listeria monocytogenes in the production environment and on carcasses and meat cuts. Its occurrence may result in meat contamination, leading to economic losses and a potential risk for public health, as evidenced by recent outbreaks. Therefore, there is a need for scientifically based knowledge to provide these companies with strategies to control better this foodborne pathogen, whose introduction and variable contamination routes remain challenging to characterize.

This work aims at filling knowledge gaps on the subject. To this end, we first mapped the L. monocytogenes occurrence on carcasses in 7% of the Belgian slaughterhouses. The pathogen was repeatedly recovered from beef (47%) and pig (20%) carcasses just before cooling. Microbial contamination of carcasses differed substantially according to the location on the carcass. Ventral and anterior sites were more contaminated on pig carcasses, probably associated with inadequate decontamination in the unclean zone or potential post-contamination during evisceration. As for cattle carcasses, contaminated sites such as hind leg, flank, brisket, and foreleg suggest pathogen transfer during (manual) dehiding practices. Moreover, in one of the slaughterhouses, the beef carcasses were contaminated entirely with a specific strain (belonging to CC9), suggesting a persistent source of contamination along the slaughter line.

We further investigated contamination routes in beef slaughterhouses by sampling tracked animals/carcasses and the environment. The hides were contaminated with L. monocytogenes in 93% of the incoming animals, signifying high contamination pressure levels entering the slaughterhouses. Sporadic pathogen transfer to carcasses during dehiding was confirmed by identifying the same strains from hides, carcasses, and utensils. Still, persistence of one strain (belonging to CC9) in a carcass splitter showed a more significant problem, causing constantly repeated contamination of carcasses over more than one year.

Furthermore, we investigated the occurrence and genetic diversity of L. monocytogenes in pork cutting plants by thoroughly sampling the processing environment to identify harborage sites. Non-food contact surfaces (zone 3) were contaminated more frequently at typical harborage sites. Although less frequently contaminated, food contact surfaces (zone 1) were also positive for detecting this pathogen, even after cleaning and disinfection. The isolated strains showed low genetic heterogeneity. Some identical pulsotypes belonging to typical clonal complexes (CC8, CC9, CC31, and CC121) were predominant and widespread, suggesting persistence rather than occasional introduction and repeated contamination.

Lastly, we carried out challenge test studies according to the EURL Lm Technical Guidance Document for conducting shelf-life studies on L. monocytogenes in ready-to-eat foods. The growth potential was determined in pork chops and minced pork under packaging and storage conditions that mirror retail and home conditions. Pork chops did not support the growth of the pathogen (<0.5 log CFU/g) under the applied conditions (air and MAP). Substantial growth of L. monocytogenes (>0.5 log CFU/g) was obtained in minced pork for air-packaging and MAP. However, significant intra- and inter-batch variability was observed, raising questions about how the growth potential is calculated. Additionally, the maximum growth rate in minced pork at a constant temperature of 7°C was determined in air and MAP for simulation studies. Determining the growth potential in minced pork using these models, as well as the Combase Growth model and DMRI dynamic safety model, gave overestimations, especially for MAP. In our view, the models used do not sufficiently consider the raw pig matrix dynamics, which may have an inhibitory effect on the growth of L. monocytogenes. On the other hand, the experimentally high variability impedes the utility of purely deterministic predictive microbiology models in these raw pork products and probabilistic approaches are essential to reflect reality.

In conclusion, this research has provided insights on contamination sources and routes, critical points, persistence, and growth of this pathogen to help the sector develop concrete action plans and better control the problem.