Taran Skjerdal

Influence of storage temperature on gene expression and virulence potential of Listeria monocytogenes strains grown in a salmon matrix.

Little is understood about the impact of environmental conditions on the virulence plasticity of Listeria monocytogenes strains grown in food. In this report, we monitored changes in the virulence properties of one high virulent (CCUG 3998) and one low virulent (442) L. monocytogenes strains grown on raw salmon (Salmo salar L.). The effect of temperature exposures (0 degrees C, 4 degrees C and 20 degrees C) on the expression levels of virulence genes (hlyA, actA, inlA and prfA), invasion into Caco-2 cells and in vivo mouse infection was analysed. Our results showed that L. monocytogenes virulence genes are differentially expressed when salmon is stored at different temperatures. Of the four virulence genes, the transcript levels for inlA were strongly affected, which correlated with the strains virulence capacity as assessed by Caco-2 cells. In contrast to CCUG 3998, the virulence of strain 442 was altered with tested conditions. This strain maintains its low virulence status as far as salmon is stored at lower temperatures, but increases its virulence at higher temperatures. These results lead to the indication that exposure to abuse temperature conditions might influence the virulence potential of low pathogenic L. monocytogenes strains in salmon.

Hurdle technology to ensure the safety of seafood products.

The microbial safety and stability of most food, are based on an application of preservative factors called hurdles. Each hurdle implies putting microorganisms in a hostile environment, which inhibits their growth or causes their death (Leistner, 2000). Some of those hurdles have been empirically used for years to stabilize meat, fish, milk and vegetables. This sometimes leads to completely different product with its own new taste characteristics. Examples of hurdles in marine products are salt (salted cod, klipfish), smoke (cold or hot smoked salmon, herring), acids (marinated products, pickles), temperature (high or low), fermentative microorganisms (traditional Asian sauces) and more recently redox potential (vacuum-packed products). Those preservative factors have been studied for years, but a large amount of potential hurdles for food have already been described including organic acids, bacteriocins, chitosan, nitrate, lactoperoxidase, essential oil, modified atmosphere packaging... , as well as novel decontamination technologies such as microwave and radio frequency, ohmic and inductive heating, high pressure, pulsed electric field, high voltage arc discharge, pulsed light, oscillation magnetic field, ultraviolet light, ultrasound, X-ray, electrolyse NaCl water, ozone... (Kim et al, 1999 ; Weber, 2000 ; Mahmoud et al, 2006). Hurdles that have a positive effect by inhibiting microorganisms may have a negative one on other parameters such as nutritional properties or sensory quality, depending on their intensity. As an example, salt content in food must be high enough to inhibit pathogens and spoilage microorganisms, but not so high to impair taste. In order to lower the preservative level, the hurdle technology concept has been developed (Leistner, 1985), consisting in using combined hurdles to establish an additive antimicrobial effect, and even sometimes a synergetic one, thus improving the safety and the sensory quality of food.

The STARTEC Decision Support Tool for Better Tradeoffs between Food Safety, Quality, Nutrition, and Costs in Production of Advanced Ready-to-Eat Foods:

A prototype decision support IT-tool for the food industry was developed in the STARTEC project. Typical processes and decision steps were mapped using real life production scenarios of participating food companies manufacturing complex ready-to-eat foods. Companies looked for a more integrated approach when making food safety decisions that would align with existing HACCP systems. The tool was designed with shelf life assessments and data on safety, quality, and costs, using a pasta salad meal as a case product. The process flow chart was used as starting point, with simulation options at each process step. Key parameters like pH, water activity, costs of ingredients and salaries, and default models for calculations of Listeria monocytogenes, quality scores, and vitamin C, were placed in an interactive database. Customization of the models and settings was possible on the user-interface. The simulation module outputs were provided as detailed curves or categorized as “good”; “sufficient”; or “corrective action needed” based on threshold limit values set by the user. Possible corrective actions were suggested by the system. The tool was tested and approved by end-users based on selected ready-to-eat food products. Compared to other decision support tools, the STARTEC-tool is product-specific and multidisciplinary and includes interpretation and targeted recommendations for end-users.

Survival of Listeria monocytogenes during in vitro gastrointestinal digestion after exposure to 5 and 0.5 % sodium chloride

The food industry is under pressure to reduce the NaCl content in food, but the consequences on the ability of L. monocytogenes to survive in the human host and cause listeriosis is not known. In this study, a recently developed internationally harmonized static in vitro digestion (IVD) model was used to investigate the survival of L. monocytogenes in the gastric and intestinal phases after exposure to 5 or 0.5% NaCl. Six isolates from three Scandinavian foodborne listeriosis outbreaks, all related to NaCl containing foods, the EGDe reference strain and an EGDe mutant, deleted for the major stress regulator gene, sigB, were included. A ten-fold reduction of NaCl in the cultivation media significantly reduced the survival fraction of the EGDe strain in the IVD model while one of the clinical outbreak isolates showed a significantly increased survival fraction. Finally, the EGDe strain was able to attach and invade cultured HT-29 cells after passage through the IVD model. Altogether, these results suggest that a reduction of the NaCl content from 5 to 0.5% prior to exposure to the IVD model has the potential to cause a change in the relative survival fraction and that the effect is strain dependent.