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.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.