Listeria monocytogenes: Biofilm formation and persistence in food processing environments

Summary

Listeria monocytogenes is the causal agent of the serious foodborne illness listeriosis. Contaminated food is now regarded as the main cause of human listeriosis. Listeria monocytogenes is ubiquitous in nature and can be introduced to food processing environments through many routes where it may become established on food processing equipment and in food processing environments and subsequently contaminate products during processing. In addition, the bacterium can grow in food at refrigeration temperatures and is therefore of major concern for the food industry. For each individual processing plant a limited number of clones of Listeria monocytogenes may become established and can be persistent for years. Persistent strains grow in biofilms that protect them against environmental stress and can be isolated from surfaces after cleaning and disinfection. Persistent strains adhere to surfaces and form biofilm more readily than sporadically found strains, thus adherence to surfaces seem to be important for survival and persistence of Listeria monocytogenes in food processing environments. Listeria monocytogenes can adhere to all the materials commonly used in the food industry although adhesion seems to be less on some materials like buna-n rubber. Adhesion and biofim formation is affected by environmental conditions like temperature, pH, nutrition and presence of other bacteria. Experiments using proteomic techniques to investigate the molecular mechanisms of biofilm formation in Listeria monocytogenes have recently been performed and several proteins induced or repressed during biofilm formation have been identified. L. monocytogenes in biofilms are much more resistant to disinfection than their free living counterparts and thick complex biofilms are more difficult to remove than adhered single cells of Listeria monocytogenes. Thus it is important with daily cleaning and disinfection to avoid development of complex biofilms. Several novel techniques to avoid adhesion of L. monocytogenes have been proposed, but high costs, practical difficulties or resistance problems limit their practical use. Despite considerable research on the adhesive properties and resistance of L. monocytogenes enabling survival in the food production environments, a final solution for avoiding establishment of the bacterium has not yet been found.