Biofilms, long-term persistence of L. monocytogenes remain a challenge.

Specific genotypes of L. monocytogenes have been repeatedly isolated from food processing environments, sometimes over periods of years. The presence of these persistent strains presents a risk of product contamination, so efforts must be made to eliminate them from the plant environment.

Some studies have shown that persistent strains have better attachment capabilities or improved resistance to sanitizers. But other studies show no such relationships. The inability to identify a consistent phenotype associated with persistence may be the result of the repeated isolation of specific genotypes over time.

The inability to identify a consistent phenotype may be the result of the growth of the strain at a specific niche not targeted by plant sanitation efforts, or the ability of the strain to adapt to stress, which allows it to survive ineffective cleaning and sanitation. Another cause could be the repeated introduction of the genotype from another source.

Biofilms are an important concern. These are aggregates of bacteria encased in a structured polysaccharide matrix that they synthesize and that attaches a bacterial community to a surface. Biofilms may be pure cultures consisting of a single type of organism, or they can be a mixed community of organisms. Almost any surface is susceptible to biofilm formation.

Phenotypic characteristics that could enhance the persistence of a bacterial strain and its resistance to sanitizers include its attachment ability, its ability to form biofilm, the ability to grow in mixed culture biofilms, and its ability to adapt to stresses associated with sanitizers, heat, dehydration and starvation. The relative contribution of these characteristics to strain persistence is unknown.

Further information. Joseph Frank, Department of Food Science and Technology, University of Georgia, Office #335, Lab #350 E, Athens, GA 30602; phone: [phone]; fax: [phone]; email: [e-mail].