ARS | Publication request: In vitro multiplication of Salmonella Enteritidis on the egg yolk membrane and penetration to the yolk contents

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 29, 2008
Publication Date: September 1, 2008
Citation: Gast, R.K., Guraya, R., Bouldin, J.G., Holt, P.S. 2008. Multiplication of Salmonella Enteritidis on the yolk membrane and penetration to the yolk contents at 30 c in an in vitro egg contamination model. Journal of Food Protection. 71:1905-1909.

Interpretive Summary: Chickens infected with Salmonella Enteritidis can sometimes deposit this pathogen inside eggs, resulting in the transmission of diarrheal disease to consumers. Although Salmonella is not often deposited inside egg yolks, bacteria from the surrounding albumen might penetrate through the vitelline membrane that surrounds the yolk and begin rapid and extensive growth in the nutrient-rich interior yolk contents. Egg refrigeration halts both penetration and growth by bacteria, but a proposed national S. Enteritidis control program would allow unrefrigerated storage of eggs on farms for up to 36 hours. The present study used a laboratory egg contamination model to assess the ability of four S. Enteritidis strains to grow on the outside of the yolk membrane before (or in addition to) penetration into the yolk contents during 36 hours of incubation at 30' C. All four Salmonella strains were able to penetrate the yolk membrane during the first 12 hours of incubation. At both 12 and 36 hours of incubation, S. Enteritidis grew to significantly higher levels in association with the yolk membrane than inside the yolk contents. These results demonstrate that extensive bacterial multiplication on the yolk membrane may occur in addition to (and before) penetration into the yolk contents. This provides further support for an emphasis on rapid refrigeration of egg for protecting consumers from egg-borne transmission of S. Enteritidis infections.

Technical Abstract: Refrigeration to limit bacterial multiplication is a critical aspect of efforts to control the transmission of Salmonella enterica serovar Enteritidis (S. Enteritidis) to consumers of contaminated eggs. Although the nutrient-rich yolk interior is an uncommon location for S. Enteritidis contamination in freshly laid, naturally contaminated eggs, migration across the vitelline membrane could lead to rapid bacterial multiplication even when the initial site of deposition is outside the yolk. Multiplication on the yolk membrane (before or in addition to multiplication within the yolk contents) could be another source of increased risk to consumers. The present study used an in vitro egg contamination model to compare the abilities of four strains of S. Enteritidis to either multiply in association with the yolk membrane or migrate through that membrane to reach the yolk contents during 36 hours of incubation at 30'C. All four S. Enteritidis strains penetrated the vitelline membrane to reach the yolk contents (at an overall frequency of 11.5%) after 12 hours of incubation. The mean log10 concentration of S. Enteritidis was significantly higher in whole yolks (including yolk membranes) than in yolk contents at both 12 hours (0.818 vs. 0.167 cfu/ml) and 36 hours (2.767 vs. 1.402 cfu/ml) of incubation. These results demonstrate that S. Enteritidis multiplication on the vitelline membrane may both precede and exceed multiplication resulting from penetration into the yolk contents during the first 36 hours of unrefrigerated storage, reinforcing the importance of rapid refrigeration for protecting consumers from egg-transmitted illness