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 contaminati on. Three hundred and forty samples were collected and analysed for th e presence of Listeria. Sixty-one Listeria strains (L. monocytogenes, 44, and L. innocua, 17) were isolated from four varieties of cheese, c heese brines, process equipment and plant environment. The L. monocyto genes strains were recovered during the ripening and rind washing stag es 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 s howed different serovars and phagovars. Some strains with the same ser ovar and the same phagovar were isolated from cheeses and process equi pment (shelves) indicating that cheese contamination occurs during rip ening. Only one profile was found from the analysis of the ribosomal r ibonucleic acid (rRNA) gene restriction patterns of 38 L. monocytogene s strains with different serovars and phagovars. This suggests that al l L. monocytogenes strains isolated in the dairy plant could have been derived from a single ancestral group.