Growth of Salmonella enteritidis (SE) in egg contents from hens vaccinated with an SE bacterin

Vaccination of hens with Salmonella enteritidis (SE) bacterin has become an important industry management tool to reduce both the incidence of SE in flocks and the production of SE-contaminated eggs. After vaccination, antibodies to SE can be found in both the serum and egg yolks. The current study was undertaken to examine whether the antibodies deposited in eggs after vaccination would have any effect on the in vitro growth of 10 SE seeded into pooled egg contents and incubated at 37°C for 24 h. The SE inoculum grew well in egg contents from control-vaccinated hens, and a high percentage of these samples were culture positive for SE. Conversely, significantly fewer egg contents from vaccinated hens were positive for SE at weeks 2, 3, 4, and 5 post-primary vaccination and weeks 1, 2, and 4 post- secondary vaccination. After 24 h incubation, the SE inoculum grew to 10⁶- 10⁹ organisms ml^-1 in egg contents from control hens, while growing to less than 10⁴ organisms ml^-1 in egg contents from vaccinated hens. No inhibition of growth was observed for a second organism, Proteus mirabilis, in egg contents from bacterin-vaccinated hens, indicating antigen-specificity of the activity. Inhibition of SE growth was observed in egg contents from vaccinated hens diluted 1:5 in eggs from control hens. Mixed results were observed for egg contents receiving a 10-fold higher SE inoculum with significant inhibition of growth observed in one trial and minimal inhibition observed in a second trial. Supplementing the egg samples with an iron source abrogated the inhibition, suggesting that the iron-restricted environment found in eggs may play a role in the observed inhibition.