Effect of refrigeration on in vitro penetration of Salmonella enteritidis through the egg yolk membrane

Internally contaminated eggs have been implicated as leading sources of transmission of Salmonella Enteritidis (SE) to humans. Although SE is not often deposited inside the nutrient-rich yolks of naturally contaminated eggs, penetration through the vitelline membrane to reach the yolk contents could result in rapid bacterial multiplication. In previous studies, such penetration has been observed occasionally at warm temperatures during experiments with in vitro egg contamination models. The present study was conducted to determine whether refrigeration affects the frequency of in vitro SE penetration of the egg yolk membrane. After inoculation of small numbers of SE onto the outside of the vitelline membranes of intact yolks, immediate refrigeration of contaminated samples prevented the penetration of SE into the egg yolk contents during 24 h of storage. However, SE penetrated inside the yolk contents in 4% of contaminated egg samples refrigerated after 2 h of storage at 30 degrees C, 15% of samples refrigerated after 6 h of storage at 30 degrees C, and 40% of samples stored at 30 degrees C for 24 h (48 samples per treatment group). These results highlight the value of prompt refrigeration for restricting the opportunities for SE to multiply to high numbers inside the yolks of contaminated eggs.