B01007: Development and study of tests to differentiate between tolerant and sensitive isolates of Salmonella and Escherichia coli O157

Study Duration: July 1998 to June 2001

Contractor: Health Protection Agency (formerly Public Health Laboratory Service)

Even within closely related groups of bacteria, variations occur in their ability to survive exposure to environmental stresses, such as acid or high temperature. Tolerance of stressful conditions is critical for foodborne pathogens such as Salmonella or E. coli, since they may encounter a number of stresses during food processing, preparation or cooking of food by the consumer, and ingestion. Data generated using highly tolerant strains will help to accurately predict processing conditions that will ensure that no pathogens will survive. Therefore, it is important to identify the range of tolerance within groups of pathogens, in order to fully assess the risk of the organism surviving and going on to cause infection.

This study aims to identify simple, reproducible methods to quantify the stress tolerance of different strains of Salmonella and E. coli O157. Until now, comparison of published data on stress tolerance has been problematic due to differences in the experimental method used, so the effect of different methods on observed tolerance was investigated.

Tolerance to acid, heat, air-drying and peroxide on the appearance of bacterial colonies, growing on agar plates, is a useful method of differentiating between highly tolerant and more sensitive bacteria. The study demonstrated that high levels of tolerance, as indicated by these tests, often indicates that a major stress response protein (RpoS) is present in the bacterial cell. Many factors affected the observed tolerance, therefore it is important to standardise experimental conditions to allow an accurate comparison to be made. For example, the appearance of bacterial colonies was dependent on the type of agar, the pH, the temperature and the salt concentration. The level of tolerance also differed between E. coli and Salmonella. A scoring system has been developed, which quantifies the overall stress tolerance of E. coli O157 and may help with decision making during an outbreak of foodborne disease or when treating individual patients.

The findings of this study will make scientists aware of the inherent variability in stress tolerance within bacterial strains and encourage them to determine a strain�s tolerance in comparison to closely related strains, prior to performing extensive research work. The information will allow a more confident comparison between of data produced using slightly different methods, because the impact of these differences has now been explored and is better characterised. Much of the data produced here represent gaps in the published literature and will prompt further studies. For example, very little data exists on the persistence of bacteria in the environment. The assays for tolerance to air-drying and UV inactivation used in this study will prove to be useful models in future work.

Final report is available from the Agency's Information centre.
To obtain a copy, please contact the Enquiry Desk, Information Services, Food Standards Agency (tel: 020 7276 8181/8182 or email: infocentre@foodstandards.gsi.gov.uk).

Contact: For any enquiries concerning this research project, please contact the relevant Programme contact or email: science@foodstandards.gsi.gov.uk