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Listen to this siteTuesday 18 October 2005
This project aims to determine if Salmonellas with increased stress tolerance and virulence may emerge more frequently when exposed to stress encountered by bacteria from farm to fork.
Study Duration : July 1998 to June 2001
Contractor : Health Protection Agency (formerly Public Health Laboratory Services)
S. enteritidis PT4 and S. typhimurium DT104 dominate human salmonellosis (an infection caused by the Salmonella bacteria) in the UK. Salmonellas can respond to potentially stressful environments, such as changes in temperature or pH. It has been established that stress responses are linked to the ability to cause infection and may induce more permanent changes. Research has shown that some salmonellas , including S. enteritidis and S. typhimurium , are able to mutate with a high frequency and suggest that pre-exposure to stressful environments in food production might lead to the emergence of mutants which are better able to survive and cause infection. The use of milder processing conditions permits salmonellas to experience sub lethal stress, which may increase further the risk of selecting for, and allowing, the survival of more stress-tolerant and virulent mutants.
The study aims to examine whether exposure to salmonellas with altered survival or virulence characteristics may emerge as a result of exposure to certain procedures or environments common in food production. This study will concentrate on changes in temperature or pH and will determine if particular conditions pose a more significant risk.
The project looked at the effect of exposure to repeated, prolonged, or simultaneous stress on the ability of Salmonella strains to survive various hostile environments.
Strains of Salmonella enterica serovar Enteritidis PT4 and Salmonella enterica serovar Typhimurium DT104 were submitted to 15 repeated exposures to acid stress (pH 2.5) each followed by outgrowth at neutral pH (acid cycles). This resulted in a 3-fold increase in survival at pH 2.5. This increase was stable even after the strains were sub-cultured 5 times at neutral pH. The ability of the strains to survive heat or desiccation was marginally increased or unchanged, respectively. Despite an increase in acid resistance in the DT104 strain 30 after acid cycling, there was no significant change observed in its ability to invade and colonise mouse tissues, regardless of the infective dose used (105, 104 or 103 CFU per ml).
Acid cycling also resulted in a change in colony morphology. Examination of the acid-cycled strain confirmed that this change in colony morphology related to over-expression of the SEF17 fimbria both at 37ºC and 20ºC.
Changes in stress resistance were also observed in Salmonella strains exposed to prolonged stress under certain conditions. In strains exposed to chill (6ºC) for 4 weeks a loss of heat and acid resistance was observed. Incubation for 4 weeks in acid broth (pH 6 and 5) did not result in permanent changes to acid resistance.
Salmonella were also exposed to a pathway of changing environmental conditions to simulate the passage from chicken to human consumption via under cooked chicken or raw egg mayonnaise. The bacteria were exposed to a combination of low iron availability, acid or alkaline pH and temperatures ranging from 8 to 48ºC. Under these conditions Salmonella became less stress resistant after each stage of the pathway although some recovery of resistance was observed during the stage that simulated the preparation of raw egg mayonnaise.
In summary, Salmonella strains can develop increased stress resistance when experiencing intermittent exposure to stress.
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
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