Attachment of Listeria monocytogenes to materials commonly found in a food-processing environment

Listeria monocytogenes is a facultative intracellular pathogen and the causative agent of listeriosis. Contaminated food is thought to be the major vector of listeriosis. Food products that have been implicated in cases of listeriosis include poultry, meat, seafood, milk and their derivative products. Microbial contamination of food surfaces is an ongoing problem for the food industry and is a major risk to food quality and safety. The primary aim of this study is to investigate attachment of L. monocytogenes 10403S to different food processing surface materials. The mean number of cells recovered from short contact times and 2 hour contact times of coupons in bacterial culture significantly differ for each of the 18 materials examined with the exception of polypropylene. L. monocytogenes transposon mutants were generated and a cellular staining assay was used for screening, selecting for those mutants attaching to glass coverslips in low frequency. One mutant was investigated further. An ORF disrupted by transposon mutagenesis in this mutant showed 48% identity to the L. monocytogenes strain EGD alpha-mannosidase and a conserved domain belonging to the glycosyl hydrolase family (alpha-mannosidases). An enzyme assay confirmed that expression of this enzyme was reduced in the mutant. Alpha-mannosidase is part of the alginate pathway characterized in Pseudomonas aeruginosa. Alginate activity has been shown to be up-regulated during attachment of P. aeruginosa to materials (Davies et al., 1993) and is implicated in bacterial attachment to surfaces and biofilm formation. We therefore suggest a hypothesis that interrupting an ORF coding for an alpha-mannosidase enzyme disrupts the alginate synthesis pathway responsible for extracellular polysaccharide (EPS) production and hence inhibits attachment to food processing surfaces. Further experiments such as site-directed mutagenesis are required to test this hypothesis.