Submitted to: Poultry Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/13/2004
Publication Date: 4/1/2005
Citation: Gast, R.K., Holt, P.S., Murase, T. 2005. Penetration of salmonella enteritidis and s. heidelberg into egg yolks in an in vitro contamination model. Poultry Science. 84:621-625 Interpretive Summary: Eggs that contain Salmonella in their edible contents can transmit diarrheal disease to consumers. Although chickens infected with Salmonella do not deposit this pathogen inside egg yolks very often, bacteria from the surrounding albumen might penetrate through the membrane that surrounds the yolk, resulting in rapid and extensive Salmonella growth in the nutrient-rich interior contents of the yolk. The present study used a laboratory egg contamination model to assess the ability of Salmonella strains to penetrate through the yolk membrane and multiply inside yolks. Strains of S. enteritidis or S. heidelberg were initially inoculated onto the outside of the membranes of separated yolks that were then recombined with albumen in plastic tubes. All of the Salmonella strains were found to capable of entering enter the yolk contents (in 10% to 25% of the experimentally contaminated eggs) during 24 hours of incubation at 30° C (86° F). When laying hens were infected with these same Salmonella strains and new isolates were obtained from eggs laid by the chickens, the new isolates penetrated through yolk membranes in the laboratory contamination model at significantly higher frequencies than the original parent strains. These results support an emphasis on prompt refrigeration of eggs in Salmonella control programs to minimize the risk that pathogens will grow to higher (and more dangerous) levels after penetrating into the yolks of contaminated eggs.
Technical Abstract: Eggs that harbor Salmonella in their edible contents pose a significant risk of transmitting disease to consumers. Although Salmonella deposition inside yolks does not usually occur at a high frequency in naturally contaminated eggs, bacterial penetration through the vitelline membrane could lead to rapid and extensive multiplication in the nutrient-rich yolk contents. The present study used an in vitro egg contamination model to assess the ability of Salmonella strains to penetrate through the vitelline membrane and multiply inside yolks. An S. enteritidis strain and two S. heidelberg strains, initially inoculated onto the outside of the vitelline membrane, were all able to enter the yolk contents (at frequencies ranging from 10 to 25% of experimentally contaminated eggs) during 24 hours of incubation at 30° C.. Variants of these parent strains, obtained by in vivo passage into eggs laid by infected hens, penetrated through the yolk membrane at significantly higher frequencies. These results demonstrate the importance of prompt refrigeration for minimizing the possibility that pathogens such as S. enteritidis and S. heidelberg will multiply to more dangerous levels after penetration into yolks of contaminated eggs.