Submitted to: International Journal of Poultry Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/21/2014
Publication Date: 2/4/2014
Citation: Gast, R.K., Guraya, R. 2014. Continuing multiplication of Salmonella Enteritidis strains in egg yolk during refrigeration at 7.2° C. International Journal of Poultry Science. 12:622-627.
Interpretive Summary: Most human illnesses caused by Salmonella Enteritidis (SE) have been attributed to the consumption of contaminated eggs produced by infected laying hens. To address this problem, risk reduction programs have been implemented for commercial egg production. In the USA, federal regulations require eggs to be refrigerated at 7.2° C or lower in order to prevent the growth of bacteria to high levels (which would be more likely to cause disease). However, it is possible that some microbial growth will continue inside warm eggs after they are transferred into refrigerated storage. The present study compared the abilities of 8 SE strains to continue growing in experimentally contaminated egg yolk samples during the first 24 hours after they were transferred from warm to refrigeration temperatures. Small numbers of SE were introduced into egg yolk samples which were first incubated at a warm temperature to encourage bacterial multiplication, and then transferred to refrigeration at 7.2° C. The number of SE cells in each sample were counted after the incubation step and after refrigeration. All 8 SE isolates grew to high levels during 16 hours of warm temperature incubation, and 3 of these strains continued growing to significantly higher levels during the 24 hours of refrigeration. These results support the importance of prompt egg refrigeration for minimizing the number of SE cells in marketed table eggs, although refrigeration at 7.2° C may not immediately or completely halt the growth of all strains.
Technical Abstract: The continuing attribution of human illness caused by Salmonella Enteritidis (SE) to the consumption of contaminated eggs has led to widespread implementation of risk reduction programs for commercial egg production. Prompt refrigeration of eggs to prevent bacterial multiplication to dangerously high levels is a prominently applied disease control option. Federal regulations in the USA require eggs to be held at ambient temperatures of 7.2° C or lower. However, microbial growth may not cease immediately inside warm eggs after transfer to refrigerated storage. The present study compared the abilities of 8 SE strains (of 4 phage types) to continue multiplying in experimentally contaminated egg yolk samples during the first 24 h after transition from warm to refrigeration temperatures. After 15-ml samples of egg yolk (n = 12/strain) were inoculated with 10 CFU/ml of SE, they were incubated at 37° C for 16 h to encourage bacterial multiplication and then transferred into refrigeration at 7.2° C for 24 h. Bacterial cell concentrations were determined following 37° C incubation and again after both 8 and 24 h at 7.2° C. All 8 SE isolates multiplied significantly from the initial inoculum level during 16 h of incubation, reaching an overall mean of log10 8.790 CFU/ml. After refrigeration, the observed mean values for cell concentrations in yolk samples were log10 8.780 CFU/ml at 8 h and log10 8.849 CFU/ml at 24 h. For 3 of 8 strains, a significant (P < 0.05) increase in cell concentrations in egg yolk occurred during 24 h of refrigeration. These results support the importance of prompt egg refrigeration for minimizing the number of SE cells in marketed table eggs, although refrigeration at 7.2° C may not immediately or completely arrest multiplication by all strains.