Assessing the frequency and consequences of Salmonella enteritidis deposition on the egg yolk membrane

The site of deposition of Salmonella enteritidis in eggs could influence the extent to which this pathogen multiplies before refrigeration achieves growth-inhibiting internal temperatures. The first part of this study sought to determine whether S. enteritidis inoculated onto the exterior (vitelline) membrane surface of egg yolks was able to penetrate into and multiply within the yolk contents. When 10(2) cfu of S. enteritidis was inoculated onto the exterior surface of intact egg yolks, multiplication within the interior yolk contents occurred in 10% of samples after 6 h of incubation and in 75% of samples after 24 h at 25 C (reaching mean levels of about 10(4) cfu/mL) but in only 20% of samples incubated for 72 h at 15 C. The second part of this study applied an oral infection model in laying hens to establish the relative proportions of contaminated eggs in which S. enteritidis deposition was associated with the yolk membrane or was found inside the yolk contents. Although approximately 4.3% of egg yolks were positive for S. enteritidis when both yolk contents and membranes were sampled, only about 0.5% of samples of yolk contents (without membranes) were positive. Although deposition of S. enteritidis within egg yolks appears to occur infrequently, rapid refrigeration of eggs is necessary to prevent the penetration of S. enteritidis into and multiplication within egg yolks.