Key messages

  • There is no difference between types of Salmonella and their ability to persist on eggs.
  • Lowering the storage temperature of eggs to less than 4 °C can reduce the risk of Salmonella surviving on the egg surface.
  • Salmonella can grow very quickly on eggs and egg contents, so eggs and egg contents should be kept below 7 °C.

Principal researchers: Dr Narelle Fegan (CSIRO) and Dr Geoff Hogg (University of Melbourne)

Summary

Salmonellosis in Australia has been linked to eggs and egg products. Outbreaks are often caused by specific serotypes and phage types of Salmonella enterica subsp. enterica. One such phage type is Salmonella Typhimurium phage type 135 (PT135).

This project was conducted by Catherine M McAuley, Lesley L Duffy and Narelle Fegan (CSIRO Animal, Food and Health Sciences), and John Coventry and Geoff Hogg (Microbiological Diagnostic Unit Public Health Laboratory, University of Melbourne).

The project team investigated the hypothesis that Salmonella serotypes that cause egg-related outbreaks might have particular traits that provide them with an enhanced ability to survive in and on eggs under Australian egg production conditions.

Salmonella strains that can attach to egg surfaces in high numbers and survive on egg surfaces for long periods are more likely to contaminate egg contents, where they may increase in number to a level that can cause human disease. The traits that were investigated in this project therefore included the ability of Salmonella to grow in egg contents, attach to egg surfaces and survive on egg shells.

Three different isolates of Salmonella were used for this work, including Salmonella Typhimurium PT135 (associated with an egg outbreak in Australia), a reference isolate of Salmonella Typhimurium and an isolate of a serotype that has never been associated with eggs, S. enterica subsp. salamae serotype II 1,4,12,27:b:[e,n,x] (Salmonella Sofia).

Experiments were conducted to look at the growth of each of these isolates in egg contents (egg yolks, egg whites and whole eggs) at three different temperatures:

  • 15 °C, which is the Australian Egg Corporation Limited’s recommended egg storage temperature
  • 22 °C, which represents ambient temperature
  • 37 °C, which is the optimum growth temperature for Salmonella.

All isolates grew rapidly (with no significant differences between isolates) in egg yolks, reaching stationary phase (108–109 CFU/mL–1) in 10 hours at 37 °C, 26 hours at 22 °C and 3 days at 15 °C.

The time to reach stationary phase in whole egg was at least 25 per cent longer than in egg yolks, while growth in egg white was very limited and cell numbers often decreased over time.

The attachment of Salmonella to egg shells was determined at two different times (1 minute and 20 minutes) and three different temperatures: 4 °C to represent refrigeration temperature, 22 °C and 42 °C, which is a temperature used for washing eggs and is also the internal temperature of a chicken.

All isolates attached to the egg shells at levels between 104 and 105 CFU/mL of egg wash, with few significant differences observed between the isolates under the different conditions. The number of cells attached to the eggs was higher after 20 minutes than after 1 minute, and numbers of attached cells of Salmonella Sofia tended to be slightly higher as the temperature increased. When Salmonella were inoculated onto eggs that were then stored at 4 °C and 22 °C, they did not survive beyond 4 weeks.

Overall, the results from this work did not confirm the hypothesis that the egg outbreak–associated Salmonella Typhimurium PT135 possessed traits that enhance its ability to grow in egg contents, survive on eggs or attach to egg shells. It is likely that factors other than those investigated in this project contribute to the reasons that this particular serotype caused an outbreak associated with eggs.

Although no differences were found in the ability of different Salmonella serotypes to grow in egg contents or attach to and survive on egg surfaces, a few key findings were identified that may provide useful information to the industry and public health authorities. These include the following:

  • Salmonella can grow very rapidly in egg contents, particularly at higher temperatures; therefore, storage of eggs and egg contents should occur at temperatures less than 7 °C (the minimum temperature for growth of Salmonella in food)
  • For at least some Salmonella serotypes, attachment to eggs was slightly less at lower temperatures; therefore, holding eggs at lower temperatures and removal of any contamination as soon as possible may help reduce the number of cells of Salmonella that can attach to egg surfaces.
  • Storing eggs at refrigeration temperatures (4 °C) will limit the risk of any Salmonella-contaminating egg surfaces surviving for more than 4 weeks.

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