Pathways of Listeria monocytogenes contamination in the meat processing industry

One hundred and thirty-three isolates of Listeria monocytogenes from deboned fresh meat, production environment, cold cuts from five meat processing plants and from one plant producing cured dried sausages, were characterized using multilocus enzyme electrophoresis. On the basis of electrophoretically demonstrable allelic variation at 21 enzyme loci, 21 electrophoretic types (ETs) were distinguished. Analysis of the genetic relationships among the 21 ETs revealed two distinct clusters: Cluster A and Cluster B. With the exception of two isolates from one plant, all isolates from deboned fresh meat belonged to Cluster B. During processing of cold cuts, however, isolates belonging to Cluster A became more frequent, and only one of the 37 isolates from cold cuts belonged to Cluster B. In contrast, six of the nine isolates from cured dried sausages had ETs in Cluster B. One clone of Cluster A, ET-6 was isolated from cold cuts in four of six plants. This is one of the ETs most frequently recovered from patients in Norway. Isolates of ET-6 were further characterized using restriction fragment length polymorphism (RFLP) analysis of chromosomal DNA. Six distinct restriction patterns were distinguished among the 44 ET-6 strains. In one plant, four different RFLP patterns could be identified. Two clone variants seemed to have colonized different areas in this plant for at least four years. However, in each of the other plants, all ET-6 isolates had the same RFLP patterns.