Modification of a virulence-associated phenotype after growth of Listeria monocytogenes on food

To assess the effect of different foods, which have been implicated or not in cases of listeriosis, on the in vitro virulence-associated phenotype level of different Listeria monocytogenes strains. The virulence-associated phenotype level of L. monocytogenes was studied with the in vitro cell test b...

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Published in:Journal of applied microbiology 2006-08, Vol.101 (2), p.300-308
Main Authors: Midelet-Bourdin, G, Leleu, G, Copin, S, Roche, S.M, Velge, P, Malle, P
Format: Article
Language:English
Subjects:
Animals
Bacteriological Techniques
Biological and medical sciences
cell culture
food
Food Handling
Food Microbiology
Fundamental and applied biological sciences. Psychology
Humans
Life Sciences
Listeria monocytogenes
Listeria monocytogenes - growth & development
Listeriosis - transmission
Meat - microbiology
Microbiology
Microbiology and Parasitology
Milk - microbiology
Phenotype
potted minced pork
salmon
Salmon - microbiology
Swine
Virulence
virulence‐associated phenotype
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Summary:To assess the effect of different foods, which have been implicated or not in cases of listeriosis, on the in vitro virulence-associated phenotype level of different Listeria monocytogenes strains. The virulence-associated phenotype level of L. monocytogenes was studied with the in vitro cell test based on a plaque-forming assay with a human adenocarcinoma cell line (HT-29) monolayer. Three strains of L. monocytogenes were grown in preparations (homogenate, 1-μm filtrate or 0·2-μm filtrate) of different food extracts ['rillettes' (potted minced pork), milk, raw salmon and cold-smoked salmon] or in a control medium, brain heart infusion (BHI). The bacterial suspensions grown in food extracts or in BHI at 37°C were diluted with their growth medium (food extract or BHI) or with minimum essential medium before seeding on confluent HT-29 cell monolayers. Filtration of food extracts had no significant effect on the plaque numbers formed by the bacteria. A significant decrease in the plaque numbers was noted for the three strains when they grew in the rillettes extracts, compared with the other food extracts and BHI. The levels of in vitro virulence-associated phenotype of the strains after growth in the rillettes extract were similar to or lower than that of the hypovirulent internal reference strain L. monocytogenes 442. After growth in milk and cold-smoked salmon, the impact on virulence-associated phenotype depended on the strain. In contrast, plaque-forming assay indicated increased virulence-associated phenotype when the strains were switched from a nutrient-rich medium (food extract or BHI) to a minimum essential medium. In vitro virulence-associated phenotype level of the studied strains grown in BHI or cold-smoked salmon was the same as the control virulent strain EGD. In contrast, the nutrients present in rillettes may therefore substantially reduce the number of plaques but not the growth of L. monocytogenes. The utilization of minimum essential medium as diluent attenuates changes the effect of the food extract on virulence-associated phenotype in vitro. In the experimental design of this study, we showed that the nature of the food could affect the in vitro virulence-associated phenotype level of L. monocytogenes.
ISSN:1364-5072
1365-2672
DOI:10.1111/j.1365-2672.2006.02925.x