ARS | Publication request: Multiplication of Salmonella Enteritidis in egg yolks after inoculation outside, on, and inside vitelline membranes and storage at different temperatures

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 24, 2010
Publication Date: October 1, 2010
Citation: Gast, R.K., Guraya, R., Guard, J.Y., Holt, P.S. 2010. Multiplication of Salmonella Enteritidis in egg yolks after inoculation outside, on, and inside vitelline membranes and storage at different temperatures. Journal of Food Protection. 73:1902-1906.

Interpretive Summary: Chickens infected with Salmonella Enteritidis (SE) sometimes deposit this pathogen inside eggs, which can then transmit diarrheal disease to consumers. Although SE deposition inside egg yolks is uncommon, bacteria from the surrounding albumen can penetrate through the vitelline membrane surrounding the yolk and grow rapidly in the nutrient-rich yolk interior. Egg refrigeration halts both penetration and growth by bacteria, but a new national SE control program allows unrefrigerated storage of eggs on farms for up to 36 hours. The present study used a laboratory egg contamination model to assess SE growth in egg yolks following bacterial introduction at three different locations and storage at a range of temperatures. For all three initial contamination locations (inside yolks, on the exterior surface of yolk membranes, and in the adjacent albumen), the final levels of SE in yolks increased significantly with increasing storage temperatures. At all storage temperatures, SE growth was greatest after introduction inside yolks and least after introduction into the adjacent albumen. The results of this study demonstrate that substantial growth of SE in yolks can occur during the first day of storage, even when contamination initially occurs outside of the yolk. Moreover, the risk of bacterial growth is higher at warmer storage temperatures. This provides further support for the importance of prompt egg refrigeration for protecting consumers against egg-borne transmission of SE.

Technical Abstract: Prompt refrigeration to restrict bacterial growth is important for reducing egg-borne transmission of Salmonella enterica serovar Enteritidis (SE). The nutrient-rich yolk interior is a relatively infrequent location for initial SE deposition in eggs, but migration across the vitelline membrane can result in rapid bacterial multiplication during storage at warm temperatures. The objective of the present study was to measure the multiplication of SE in yolks after introduction at three different locations and subsequent storage at a range of temperatures. Using an in vitro egg contamination model, approximately 100 CFU of SE were inoculated either inside yolks, onto the exterior surface of vitelline membranes, or into the adjacent albumen. After storage of samples from each inoculation group at 10°, 15°, 20°, and 25° C for 24 h, SE was enumerated in yolks. For all three inoculation locations, the final SE levels in yolks increased significantly with increasing storage temperatures. At all storage temperatures, significant differences in SE multiplication were observed between inoculation sites (yolk inoculation > vitelline membrane inoculation > albumen inoculation). At 25° C, final log10 SE concentrations of 7.76 CFU/ml (yolk inoculation), 2.01 CFU/ml (vitelline membrane inoculation) and 0.76 cfu/ml (albumen inoculation) were attained in yolks after storage. These results demonstrate that, even when the initial site of SE deposition is outside the egg yolk, substantial multiplication supported by yolk nutrients can occur during the first day of storage and the risk of bacterial growth increases at higher ambient storage temperatures.