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Final Report Summary - LAGSAL (Effect of sublethal thermal injury on the survival and re-growth of Salmonella enteritidis. Model development and application to eggs products)

LAGSAL. Effect of sublethal thermal injury on the survival and re-growth of Salmonella Enteritidis.
Salmonella enterica serovar Enteritidis is the cause of a worldwide increase in human salmonellosis. During the last three decades Salmonella Enteritidis was involved in a number of food poisoning outbreaks mainly associated with the consumption of meats and egg products. In recent years there has been an increase in consumer demand for fresh, minimally processed foods, but foodborne disease linked to ready-to-eat food has also increased. The Commission Regulation (EC) No. 2073/2005 and No. 1441/2007 on microbiological criteria for foodstuffs has established a series of food safety rules for ready to eat products. An increased interest in “hurdle technology” has been observed because of the increased market demand for minimally processed foods. Hurdle technology employs the combinations of various antibacterial treatments to limit the growth of spoilage bacteria, improving the microbial safety and maintaining the sensory and nutritional quality of food. Among these hurdles, mild heat treatment, low temperature, water activity, acidity, etc have been used for centuries. However, these treatments could leave cells damaged, but not inactivated, so they may be able to grow in favourable environmental conditions. Factors such as temperature of incubation and culture medium influence the capacity of cells to repair heat damage. The objective of this study was to determine the effect of adding NaCl during the sub-lethal heat treatment of Salmonella on adaptation to the subsequent growth environment. We used automated optical density (OD) measurements to quantify the variability of the cells response to stress. The results were validated with plate count measurements and micro-array data which showed significant changes at the molecular level.
Stationary phase cultures of Salmonella enterica subs. enterica serovar Enteritidis phage type 4 were used in the experiments. Tubes containing 10 ml of tryptone soya broth plus 0.3% yeast extract (TSYB) with NaCl concentrations of 0 or 8% (w/w) were used as heating media. The heat treatments were carried out in a water bath at 60°C for 30s. Tubes were submerged in the water bath to preheat and then injected with 100 µl cell suspension directly into the liquid using a precision syringe fitted with a long sterile needle. After the heating time, the tubes were removed from the bath and cooled rapidly in ice water. Appropriate dilutions of heated samples were inoculated into five tubes of each recovery medium. Recovery media consisted of TSYB containing NaCl concentrations from 0 to 9% (w/w). Tubes were incubated at 30°C. OD, plate count and micro-arrays measurements were some of the techniques used in this study.

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