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

Abstract

Aim: 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. Methods and Results: 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. Conclusions: 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. Significance and Impact of the Study: 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.

Comparative Analysis of the Exoproteomes ofListeria monocytogenesStrains Grown at Low Temperatures

“…The importance of extracellular proteins in environmental adaptation has been reported in L. monocytogenes (Midelet-Bourdin et al, 2006;Soni et al, 2011). The temperature of 11°C was chosen as one at which growth is not excessively slow, but also because it is an abusive refrigeration temperature occurring more often than would be expected in the food chain (including in the consumer's home).…”

“…Refrigeration is a widespread process used in the food chain to extend product shelf-life as it is the most common hurdle to microbial growth, but L. monocytogenes is able to grow at low temperatures (Tasara and Stephan, 2006). The bacterium extracellular proteome or exoproteome, an important subset of the total proteome, is marked by its functional nature, undergoing variations and adaptations according to the bacterial surrounding conditions in each moment (Conte et al, 2000;Lee and Schneewind, 2001;Midelet-Bourdin et al, 2006). Altered expression of multiple cytoplasmic proteins was detected in a strain of L. monocytogenes grown at low temperature (Cacace et al, 2010).…”

Galleria mellonella as a Model System for Studying Listeria Pathogenesis

“…Of the six species that comprise the genus Listeria, only L. monocytogenes and L. ivanovii are pathogenic and cause disease, while strains of the species L. innocua, L. welshimeri, L. seeligeri, and L. grayi are generally considered to be nonpathogenic (26). L. monocytogenes is a major foodborne pathogen, and listeriosis is an invasive disease that in its severest form can lead to meningitis, meningoencephalitis, septicemia, and abortions (38). Listeriosis occurs primarily in pregnant women, newborn infants, and the elderly as well as in immunocompromised patients, with a mortality rate of about 30% (22,36).…”

Coincidental Loss of Bacterial Virulence in Multi-Enemy Microbial Communities

“…Moreover high virulence in Salmonella typhimurium can be costly in terms of reduced growth in the outside host environment because of the expression of virulence factor (type III secretion system) in a non-host environment [44]. The nutritional conditions of the bacterial growth environment can also significantly affect bacterial metabolism and the expression of virulence factors [45]–[47]. For example, it has been found that the virulence of the pathogenic fungi was negatively correlated to the carbon-to-nitrogen (C∶N) ratio of the culturing medium [48]–[50].…”