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An Assessment of Population Changes in Salmonella enteritidis, and the Emergence of Strains with Altered Properties During Food Processing

University of Reading
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This research project aims to establish whether exposure of S. enterica serovar Enteritidis to stresses such as heat or acid treatment gives rise to more resistant types.

Work in this project was co-ordinated with a similar project at the Exeter Public Health Laboratory. It was decided that Reading would concentrate on heat stress and Exeter would concentrate on acid. The original workplan at Reading was divided into two areas:

  • investigating the selective effect that repeated cycles of stress treatment would have on a mixed culture of many different Salmonella enterica serovar Enteritidis strains
  • investigating changes in resistance of single strains under the same conditions. The first part required the development of a high-resolution genetic fingerprinting method, amplified fragment length polymorphism (AFLP), that would enable individual strains within the population to be identified.

Although initial results with this method were very promising, problems of reproducibility were encountered. While this work was in progress, a report appeared in the literature describing the application of AFLP to Salmonella enterica serovar Enteritidis. In view of this, more attention was devoted to investigating the development of enhanced heat resistance in individual strains and in assessing the extent and practical significance of the variation in resistance to heat and other stresses seen among natural isolates of Salmonella enterica serovar Enteritidis.

More information
The consumer preference for foods that are fresher-tasting and more natural has led to a reduction in the levels of preservatives in some traditional foods, and the development of newer types of product that rely on combinations of mild processes (e.g. mild heat plus refrigerated storage). Great care is taken to ensure the safety of such foods. However, there is concern that mild treatments which do not kill all bacteria in food may eventually lead to the emergence of more robust strains better able to withstand heat, acid, drying and other processes used in food preservation. If such robust strains were to arise among foodborne pathogens, they would create problems for food manufacturers and might pose a threat to health.

At present there is no evidence to suggest that foodborne pathogens are becoming more resistant to adverse conditions. Nevertheless it has recently become apparent that natural isolates of Salmonella and Listeria monocytogenes show considerable variation in stress resistance and in virulence. Such strain variation is also relevant to the newer methods of food preservation. For example, some strains of Escherichia coli O157 are much more resistant to inactivation by high hydrostatic pressure than others.

The extent and basis of such strain variations in resistance are poorly understood and this has obvious practical implications in setting guidelines for safe preservation processes and in risk assessment. This project aims to establish if exposure of Salmonella enterica serovar Enteritidis to stresses such as heat or acid treatment gives rise to more resistant types either by selection of robust strains that were already present in mixed populations of natural isolates or by selection of spontaneous mutants arising from 'normally resistant' strains.

Find more about this project and other FSA food safety-related projects at the Food Standards Agency Research webpage.

Funding Source
Food Standards Agency
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