Listeria monocytogenes in pork slaughtering and cutting plants: use of RAPD, PFGE and PCR–REA for tracing and molecular epidemiology

In order to determine the origin of pork cuts contamination by Listeria monocytogenes, 287 isolates, collected from five French pork slaughtering and cutting plants, from live pigs to pork cuts, were characterised using three molecular typing methods: random amplification of polymorphic DNA (RAPD) carried out with five different primers, genomic macrorestriction using ApaI with pulsed-field gel electrophoresis (PFGE) and a PCR–restriction enzyme analysis (PCR–REA) based on the polymorphism existing within the inlA and inlB genes. Results obtained from RAPD and PFGE were closely related and distinguished respectively 17 RAPD types (r1–r17) and 17 PFGE types (a1–a17) among the 287 isolates, whereas the PCR–REA analysis only yielded two profiles (p1 and p2). Considering the combined results obtained with the three molecular typing methods, 19 Listeria monocytogenes genotypes (1–19) were distinguished. Serotyping led at least four serotypes being distinguished: 1/2a, 3a, 1/2c and 3c. The application of genotyping identified the predominance of a Listeria monocytogenes strain of type (1) and other very closely related ones (5, 9, 10, 12, 13, 14, 16 and 19) which were present on pork as well as in the environment within the five investigated plants. This study also pointed out the presence of these closely related Listeria monocytogenes strains over a 1-year period in the environments of two plants, even after cleaning and disinfection procedures. This highlights the possibility for some Listeria monocytogenes strains to persist in pork processing environments and raises the problem of the efficiency of cleaning and disinfection procedures used in pork slaughterhouses, chilling and cutting rooms.