Prolonged contamination of a dairy with Listeria monocytogenes

From 1988 to 1995, strains of Listeria monocytogenes serovar 3b were repeatedly isolated from the environment in a Scandinavian dairy and from cheeses manufactured in the dairy. Ten isolates from the dairy environment, and cheeses, were studied using restriction enzyme analysis with pulsed-field gel electrophoresis. In addition, eight strains of the same serovar obtained from other sources were analysed for comparative purposes. The restriction profiles of all ten L. monocytogenes strains isolated from the dairy were identical using the enzymes ApaI, SmaI and AscI, respectively. The strains from the other sources each showed unique combinations of the three restriction enzyme analysis profiles. The results indicate that a clone of L. monocytogenes can survive in a food industry plant for at least seven years. It cannot be ruled out that L. monocytogenes entered the plant continuously during these years; however, this appears unlikely. This observation may eventually lead to strategies that will help this dairy and other food plants to find ways of eliminating this pathogen.