Persistent and nonpersistent Listeria monocytogenes contamination in meat and poultry processing plants.

Contamination analysis of persistent and nonpersistent Listeria monocytogenes strains in three meat processing plants and one poultry processing plant were performed in order to identify factors predisposing to or sustaining persistent plant contamination. A total of 596 L. monocytogenes isolates were divided into 47 pulsed-field gel electrophoresis (PFGE) types by combining the restriction enzyme patterns of AscI (42 patterns) and ApaI (38 patterns). Persistent and nonpersistent strains were found in all plants. Nonpersistent PFGE types were found mostly at one sampling site, with the processing environment being the most common location, whereas the persistent strains were found at several sampling sites in most cases. The processing machines were frequently contaminated with persistent L. monocytogenes PFGE types, and it was of concern that surfaces having direct contact with the products were contaminated. The role of the processing machines in sustaining contamination and in contaminating the products appeared to be important because the final product of several processing lines was contaminated with the same L. monocytogenes PFGE type as that found in the processing machine. The proportion of persistent PFGE types in heat-treated products was eight times higher than in the raw products, showing the importance of the persistent PFGE types as contaminants of the final heat-treated products. The contamination status of the processing lines and machines appeared to be influenced by the compartmentalization of the processing line, with poor compartmentalization increasing L. monocytogenes contamination. The separation of raw and post-heat treatment areas seemed especially important in the contamination status of post-heat treatment lines.

Transfer of persistent Listeria monocytogenes contamination between food-processing plants associated with a dicing machine.

The possibility of the transfer of persistent Listeria monocytogenes contamination from one plant to another with a dicing machine was evaluated, and possible reasons for persistent contamination were analyzed. A dicing machine that diced cooked meat products was transferred from plant A to plant B and then to plant C. After the transfer of the dicing machine, L. monocytogenes PFGE type I, originally found in plant A, was soon also found in plants B and C. This L. monocytogenes PFGE type I caused persistent contamination of the dicing lines in plants B and C. The persistent L. monocytogenes strain and three nonpersistent L. monocytogenes strains found in the dicing line of plant C were tested for adherence to stainless steel surfaces and minimal inhibitory concentrations of a quaternary ammonium compound and sodium hypochlorite, disinfectants widely used in the dicing lines. The persistent strain showed significantly higher adherence to stainless steel surfaces than did the nonpersistent strains. The minimal inhibitory concentrations of sodium hypochlorite were similar for all strains, and the minimal inhibitory concentrations of the quaternary ammonium compound for three of the L. monocytogenes PFGE types, including the persistent PFGE type, were high. All persistent L. monocytogenes PFGE type I isolates were found in an area with high hygienic standards, with the dicing machine being the first point of contamination. These observations show that the dicing machine sustained the contamination and suggest that the dicing machine transferred the persistent L. monocytogenes PFGE type from one plant to another.