Optimization of iron supplementation for enhanced detection of Salmonella enteritidis in eggs

Mixed raw egg contents were inoculated with approximately 10 CFU of Salmonella Enteritidis and supplemented with 0 to 7 mg of FeSO₄ per g of egg contents. Egg contents were then incubated at 37°C, and Salmonella Enteritidis colonies were enumerated for up to 106 h. Iron supplementation significantly enhanced the growth of Salmonella Enteritidis. Within the first 24 h of incubation, the optimum iron level for Salmonella Enteritidis growth in egg contents was between 0.2 and 2 mg of FeSO₄ per g of egg contents. After 24 h of incubation at 37°C, Salmonella Enteritidis counts in eggs supplemented with 0.5 mg of FeSO₄ per g of egg contents consistently reached approximately 1 × 10⁹ CFU/ml, whereas Salmonella Enteritidis counts in eggs without iron supplementation varied from less than 5 CFU/ml to 8.4 × 10⁶ CFU/ml. A 3 by 3 factorial design was used to study the effect of type of preenrichment and level of iron supplementation on the growth of Salmonella Enteritidis in egg contents. No significant differences in Salmonella Enteritidis counts between preenrichment and nonpreenrichment treatments were observed when egg contents were supplemented with 0.5 mg of FeSO₄ per g of egg contents. It was concluded that preenrichment was not necessary for isolation of Salmonella Enteritidis from eggs. The effect of iron supplementation on the sensitivity of detection by the direct plating method was investigated. The direct plating method detected a significantly higher percentage of Salmonella Enteritidis in raw egg contents supplemented with 0.5 mg of FeSO₄ per g of egg contents (90%) than in raw egg contents without iron supplementation (63.3%).