Study of Listeria monocytogenes contamination in a dairy plant and characterization of the strains isolated.

From 1988 to 1990, a dairy plant was examined for Listeria contamination. Three hundred and forty samples were collected and analysed for the presence of Listeria. Sixty-one Listeria strains (L. monocytogenes, 44, and L. innocua, 17) were isolated from four varieties of cheese, cheese brines, process equipment and plant environment. The L. monocytogenes strains were recovered during the ripening and rind washing stages and not before, so it is likely that cheese contamination occurred at these points in the manufacturing process. The characterization of the isolated L. monocytogenes strains by serotyping and phage typing showed different serovars and phagovars. Some strains with the same serovar and the same phagovar were isolated from cheeses and process equipment (shelves) indicating that cheese contamination occurs during ripening. Only one profile was found from the analysis of the ribosomal ribonucleic acid (rRNA) gene restriction patterns of 38 L. monocytogenes strains with different serovars and phagovars. This suggests that all L. monocytogenes strains isolated in the dairy plant could have been derived from a single ancestral group.